Method for catalytic depolymerization of lignin to prepare monophenolic chemical product using zeolitic imidazolate framework derivative

Abstract

Disclosed is a method for catalytic depolymerization of lignin to prepare a monophenolic chemical product using a zeolitic imidazolate framework derivative. The method uses lignin as raw material, a zeolitic imidazolate framework derivative as catalyst, and an organic small molecule alcohol as hydrogen-donating solvent. After inert gas displacement and pressurization to 0.5-3 MPa, the reaction temperature is controlled to be 200-240°C, and stirring is performed for 2-10 h, which can achieve selective depolymerization of lignin without added hydrogen, yielding monophenolic chemical products mainly composed of 4-propyl-2,6-dimethoxyphenol. The method of the present invention is environmentally friendly, has stable performance, has highly efficient catalysis, and is highly selective, realizing selective conversion of lignin into monophenolic chemical products mainly composed of 4-propyl-2,6-dimethoxyphenol under conditions without added hydrogen.

Description

[0001] Method for preparing monophenol chemicals by catalyzing lignin depolymerization using zeolite imidazolate framework material derivatives Technical Field

[0002] The present invention relates to a method for preparing monophenol chemicals by catalyzing the depolymerization of lignin using a zeolite imidazolate skeleton material derivative, and belongs to the technical fields of efficient utilization of agricultural and forestry wastes and high-value utilization of renewable resources. Background Art

[0003] The world's growing energy demand, coupled with the non-renewable nature of traditional fossil fuels and the resulting environmental pollution, has prompted researchers to seek renewable, clean energy sources. Biomass is Earth's most abundant renewable organic carbon resource, and converting it into renewable fuels and chemicals is a key path to achieving the "dual carbon" goals and sustainable development.

[0004] Lignin is the second-largest biomass resource in reserves and the only renewable aromatic carbon resource in nature. Lignin is composed of phenylpropane structural units connected 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, in order to fully utilize the aromatic structural units in lignin, it is urgent to develop efficient and highly selective catalytic systems for the conversion of lignin to achieve high-value utilization of lignin.

[0005] The structural formula of 4-propyl-2,6-dimethoxyphenol is: , used as a flavoring and seasoning in the food industry; it is also an important organic synthesis intermediate in the preparation of various fine chemicals. For example, oxidation can produce 2,6-dimethoxy-1,4-benzoquinone, which has anti-cancer properties; demethoxylation can produce 4-propylphenol, a liquid crystal raw material, or 4-propylguaiacol, a raw material for daily fragrance formulations. Currently, the preparation of 4-propyl-2,6-dimethoxyphenol mainly uses a petrochemical route, which is characterized by harsh conditions, tedious steps, and numerous side reactions. Therefore, the search for efficient and mild alternatives to petroleum-based synthetic routes is crucial. The syringyl building block in lignin provides a natural raw material for the synthesis of 4-propyl-2,6-dimethoxyphenol.

[0006] US Patent US005807952A discloses a method for producing phenolic chemicals by pyrolyzing wood in the presence of a strong base. When potassium hydroxide is present in an amount of approximately 0.1 to 5 wt.%, the pyrolysis temperature is 400 to 600°C, and the reaction time is 1 to 3 minutes, 15 to 60% phenolic chemical product can be obtained. However, this method uses a homogeneous catalyst, which is difficult to separate and recycle, and produces a large amount of waste alkali, making it environmentally unfriendly. The harsh reaction conditions add to the challenges of its application.

[0007] Chinese invention patent application 202211391788 discloses a method for preparing monophenolic products by depolymerizing lignin using Ni4Ru2 / HZSM-5 catalysts. The introduction of ruthenium not only promotes the dispersion of metallic nickel but also enhances the hydrogenation activity of the catalytic system, thereby promoting the depolymerization of lignin. This system achieves a conversion rate of 65.7-89.9% for lignin depolymerization, and a monophenolic product yield of 6.7-18.7 wt.%. However, this catalytic system uses the precious metal ruthenium as a catalyst, and hydrogen is added during the depolymerization process, increasing the cost of catalytic conversion of lignin.

[0008] Chinese invention patent 2021113705176 discloses a method for catalyzing the hydrogenolysis of lignin using metallic nickel supported on sodium ligninsulfonate-based porous carbon. This method uses metallic nickel supported on sodium ligninsulfonate-based porous carbon as a catalyst, an organic small molecule alcohol as a reaction medium, and lignin as a base material to selectively convert lignin into monophenolic chemicals. The catalyst carrier is obtained by calcining a precursor, washing, and drying. The precursor uses sodium ligninsulfonate as a carbon source, ZnCl2 and KOH as pore-forming agents, and is obtained by adding ionized water to fully dissolve the reaction and then drying. However, the preparation process of the catalyst of this technology is complicated, and it has to go through two steps: pore formation and reduction. The pore formation process requires the use of a strong base, KOH, which is not environmentally friendly. The reduction process requires the use of high-purity hydrogen, which has energy consumption and safety issues. At the same time, the catalytically active center nickel of this technology is loaded onto the catalyst by impregnation. The interaction between the metal and the carrier is not strong enough, resulting in poor dispersion of the metal active center in the catalyst and insufficient utilization of the metal active center sites. Technical Solution

[0009] In order to overcome the shortcomings of the existing technology, the present invention aims to provide a method for catalytic depolymerization of lignin using a zeolite imidazolate framework material derivative that is environmentally friendly, stable in performance, catalytically efficient, and highly selective; in the absence of external hydrogen, lignin is selectively converted into monophenolic chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol.

[0010] The purpose of the present invention is achieved through the following technical solutions:

[0011] A method for preparing monophenolic chemicals by catalyzing the depolymerization of lignin using a zeolite imidazolate skeleton material derivative, comprising: using lignin extracted from biomass as a raw material, a zeolite imidazolate skeleton material derivative as a catalyst (Co@NC catalyst), and an organic small molecule alcohol as a hydrogen-donating solvent; the solvent is pressurized to 0.5-3 MPa after replacement with an inert gas; the reaction temperature is controlled at 200-240°C; and the reaction is stirred for 2-10 hours to selectively depolymerize the lignin into monophenolic chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol; the zeolite imidazolate skeleton material derivative is obtained by pyrolysis of a cobalt-based zeolite imidazolate skeleton material (ZIF-67) under an inert gas.

[0012] To further achieve the purpose of the present invention, preferably, the precursor is prepared by the following method: cobalt nitrate hexahydrate and 2-methylimidazole are dissolved in deionized water respectively, mixed and stirred at room temperature for 12 to 24 hours, centrifuged, and dried to obtain a ZIF-67 precursor.

[0013] Preferably, the molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 0.015-0.030:1.

[0014] Preferably, the inert gas is any one of nitrogen, argon and helium.

[0015] Preferably, the pyrolysis refers to placing the dried solid under an inert atmosphere and thermally decomposing it, and the pyrolysis process is carried out in a tube furnace; the pyrolysis temperature is 600-900° C., and the pyrolysis time is 1-4 hours.

[0016] Preferably, the lignin extraction method is as follows: placing the dried biomass raw material and the extract together in a hydrothermal reactor, heating at 100-120°C for 2-6 hours, filtering after cooling, washing the solid phase with anhydrous ethanol, combining the washing liquid with the filtrate, adding deionized water to precipitate the solid, standing for 12-24 hours, filtering, taking the solid phase, and drying to obtain a lignin solid raw material; the biomass raw material is any one of bagasse, bamboo, corn cob, poplar, pine and birch, which is crushed and sieved through 80-120 mesh.

[0017] 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 the extract per gram of biomass raw material is 10 to 20 mL.

[0018] Preferably, the hydrogen-donating solvent organic small molecule alcohol is any one of methanol, ethanol, ethylene glycol, isopropanol and butanol.

[0019] Preferably, the mass ratio of the zeolite imidazolate framework material derivative catalyst to lignin is 0.5-1.5:1.

[0020] Preferably, the monophenol 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. Advantageous Effects

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1) The catalyst preparation process of the present invention is environmentally friendly and requires no external pore-forming agent. The multi-level porous catalyst is obtained by utilizing the changes in the precursor itself during pyrolysis. Compared to using high-purity hydrogen to reduce the catalyst, the present invention utilizes the inherent reducing properties of carbon to pyrolyze the catalyst in an inert atmosphere, which is more economical and safer. Furthermore, the carbon in the precursor is used to perform a one-step pyrolysis reduction of the active metal components, which increases the interaction between the metal and the support. The cobalt metal is highly dispersed on the carbon support, allowing the cobalt active sites to be fully utilized during the catalytic process. This enables the catalyst to catalyze both solvent hydrogen production and lignin hydrogenolysis, thereby improving the performance of catalyzing lignin hydrogenolysis while achieving the goal of selective depolymerization of lignin.

[0023] 2) The present invention utilizes the unique properties of the precursor pyrolysis process to generate carbon nanotube structures in situ. Carbon nanotubes have excellent hydrogen storage capacity and a unique surface and void structure, making them the primary structure for catalytic hydrogenolysis reactions and shape-selective catalysis. Therefore, the presence of the carbon nanotube structure enhances the catalyst's adsorption of lignin and the selectivity of the main product, 4-propyl-2,6-dimethoxyphenol.

[0024] 3) The zeolite imidazolate framework material derivative catalyst used in the present 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 the active center of the catalyst, thereby enhancing the mass transfer between lignin and the active site of the catalyst, improving the performance of catalytic hydrogenolysis of lignin, and providing nano-sized space for the selective hydrogenolysis of lignin to produce 4-propyl-2,6-dimethoxyphenol as the main product.

[0025] 4) The nitrogen-doped carbon structure of the catalyst of this invention effectively modulates the catalyst's physical and chemical properties and electronic structure, promoting electronic interactions between cobalt and nitrogen. The carbon-nitrogen structure's anchoring effect on cobalt promotes the dispersion of cobalt nanoparticles, enhancing catalyst activity and achieving higher selectivity for the main product under milder conditions. It also prevents cobalt loss during the catalytic process, enhancing catalyst stability.

[0026] 5) The present invention uses lignin as a raw material, a zeolite imidazolate framework material derivative as a catalyst, and 10 mL of isopropanol as a hydrogen donor solvent. After inert gas replacement and pressure charging to 0.5-3 MPa, the reaction temperature is controlled at 200-240° C. and stirring is carried out for 2-10 hours. The lignin conversion rate is 79.8-89.4%, the yield of monophenol products is 11.0-16.1 wt.%, and 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%. After the hydrogenolysis reaction of 0.1 g of lignin is carried out using isopropanol as the hydrogen donor solvent, the lignin consumes hydrogen in the reaction and is hydrogenated into monophenol products under the action of the catalyst. The amount of hydrogen generated per 10 mL of isopropanol is 2.9-9.7 mmol.

[0027] 6) The technology used in this invention to catalyze the depolymerization of lignin to produce monophenolic chemicals using derivatives of zeolite-like imidazolate framework materials boasts renewable raw materials, a simple reaction process, mild reaction conditions, and an environmentally friendly catalyst. This technology, using derivatives of zeolite-like imidazolate framework materials, partially addresses the low lignin conversion rates, yields, and selectivity of target products associated with conventional technologies. Description of Drawings

[0028] FIG1 is an XRD spectrum of the zeolite imidazolate framework material precursor ZIF-67 in Example 1 of the present invention.

[0029] FIG2 is a nitrogen adsorption-desorption isotherm of the zeolite imidazolate framework material precursor ZIF-67 in Example 1 of the present invention.

[0030] FIG3 is a pore size distribution diagram of the zeolite imidazolate framework material precursor ZIF-67 in Example 1 of the present invention.

[0031] FIG4 is an XRD spectrum of the zeolite imidazolate framework material derivative catalyst Co@NC-800 in Example 1 of the present invention.

[0032] FIG5 is a nitrogen adsorption-desorption isotherm of the zeolite imidazolate framework material derivative catalyst Co@NC-800 in Example 1 of the present invention.

[0033] FIG6 is a pore size distribution diagram of the zeolite imidazolate framework material derivative catalyst Co@NC-800 in Example 1 of the present invention.

[0034] FIG7 is a scanning electron microscope image and an EDS mapping image of the zeolite imidazolate framework material derivative catalyst Co@NC-800 in Example 1 of the present invention.

[0035] FIG8 is an X-ray energy spectrum of the zeolite imidazolate framework material derivative catalyst Co@NC-800 in Example 1 of the present invention.

[0036] FIG9 is a gas chromatogram of the product obtained by depolymerization of organosoluble bagasse lignin catalyzed by Co@NC-800 in Example 6 of the present invention.

[0037] FIG10 is a mass spectrum of 4-propyl-2,6-dimethoxyphenol, the main product, obtained by depolymerization of organosoluble bagasse lignin catalyzed by Co@NC-800 in Example 6 of the present invention. Mode for Invention

[0038] For a better understanding of the present invention, the present invention is further described below with reference to the accompanying drawings and examples, but the embodiments of the present invention are not limited thereto.

[0039] The present invention uses the zeolite imidazolate framework material ZIF-67 as a precursor, and utilizes its ordered periodic network structure and thermal instability to obtain a Co@NC catalyst after thermal decomposition under an inert gas atmosphere. Cobalt is the active center for catalyzing hydrogen production from isopropanol and lignin hydrogenolysis. The nitrogen-doped carbon structure effectively regulates the electronic structure of the catalyst, and the multi-level pore structure promotes the diffusion between lignin macromolecules and the catalyst, thereby improving the performance of catalytic hydrogenolysis of lignin. At the same time, the carbon nanotube structure in the catalyst has a shape-selective catalytic effect, which can selectively convert the catalytic lignin into the main product 4-propyl-2,6-dimethoxyphenol, breaking through the problem of low selectivity of lignin depolymerization products catalyzed by traditional catalysts. The catalyst has strong selectivity for breaking single chemical bonds in lignin, which can effectively reduce the occurrence of side reactions and reduce the energy consumption of subsequent product separation, laying the foundation for the industrialization of high-value utilization of lignin. Compared with Chinese invention patent 2021113705176, the present invention reduces the two steps (pore formation and reduction) of the technology to a one-step pyrolysis to obtain a multi-level porous carbon-loaded non-precious metal material. The catalyst preparation process does not use strong base KOH to form pores, which is more environmentally friendly; and the catalytic system does not require external hydrogen, but uses solvents to supply hydrogen, making production safer. The lignin conversion rate, the yield of monophenolic biomass chemicals, and the selectivity of the main product 4-propyl-2,6-dimethoxyphenol yield are slightly improved under the same conditions. Specifically, the present invention discloses a method for preparing monophenolic chemicals by catalyzing the depolymerization of lignin using a zeolite imidazolate skeleton material derivative: lignin extracted from biomass is used as a raw material, a zeolite imidazolate skeleton material derivative is used as a catalyst, and an organic small molecule alcohol is used as a hydrogen-donating solvent. After replacement with an inert gas, the pressure is increased to 0.5-3 MPa, the reaction temperature is controlled at 200-240°C, and the reaction is stirred for 2-10 hours to selectively depolymerize the lignin into monophenolic chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol. The zeolite imidazolate skeleton material derivative is obtained by pyrolysis of a cobalt-based zeolite imidazolate skeleton material (ZIF-67) under an inert gas.

[0040] The characteristics and effects of the catalyst of the present invention are mainly reflected in the following aspects:

[0041] This invention uses ZIF-67 as a precursor, leveraging its ordered periodic network structure and thermal instability. After pyrolysis under an inert gas atmosphere, the carbon's inherent reducibility allows ZIF-67 to be reduced in a single step to a Co@NC catalyst, preserving some of the properties of the ZIF-67 precursor. This catalyst, utilizing the pyrolysis characteristics of ZIF-67, possesses a hierarchical pore structure, facilitating mass transfer between lignin and the catalyst.

[0042] Compared to the method of adding a carbon source and a nitrogen source and then loading an active metal, the present invention uses ZIF-67 as a precursor, and the catalyst metal-support interaction obtained by pyrolysis is stronger, and the Co-N bond of the precursor can still be retained during the pyrolysis process. The anchoring effect of Co and N enables the catalytic active center cobalt to maintain a highly dispersed state, thereby improving the activity of the catalyst. On the other hand, it also avoids the loss of Co during the catalytic process and enhances the stability of the catalyst. As shown in Example 13, after the catalyst is circulated four times, there is no obvious loss of metallic cobalt, indicating that the catalyst has good stability.

[0043] Example 1: Preparation of zeolite imidazolate framework material derivative catalyst Co@NC-800

[0044] The preparation of the zeolite imidazolate framework material derivative catalyst Co@NC-800 includes the following two steps:

[0045] (1) Preparation of ZIF-67 precursor: Weigh 1 g of 99% pure cobalt nitrate hexahydrate solid and 27.5 g of 99% pure 2-methylimidazole solid, dissolve them in 15 mL and 100 mL of deionized water, respectively, and stir for 5 min. The two solutions are mixed and stirred at room temperature for 24 h. The purple solid is separated by centrifugation, washed several times with deionized water and methanol, and dried in an oven at 60°C overnight. The purple powder ZIF-67 is obtained after grinding.

[0046] (2) Preparation of Co@NC-800 catalyst: The ZIF-67 prepared in (1) was placed in a tubular furnace and maintained at 800°C for 2 h under an inert gas atmosphere to obtain black powder Co@NC-800.

[0047] The structures of the ZIF-67 precursor and Co@NC-800 catalyst were characterized using X-ray diffraction and nitrogen physical adsorption / desorption techniques. The results are shown in Figures 1-6. The diffraction peaks of ZIF-67 in Figure 1 are consistent with those of the simulated ZIF-67 model and the XRD patterns from related studies, demonstrating the successful preparation of the ZIF-67 structure. Figure 2 shows that the adsorption isotherm of the catalyst precursor ZIF-67 is a classic Type I isotherm. The adsorption amount shows a sudden increase at low relative pressures (P / P0 < 0.45), reflecting the precursor's pore size of less than 1 nm. This is also confirmed by the pore size distribution (Figure 3), which shows a bimodal pore size distribution of ZIF-67 in the micropore range.

[0048] The Co@NC-800 catalyst was obtained by pyrolysis treatment. As shown in 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. As shown in Figure 5, the adsorption isotherm of Co@NC-800 is a type IV isotherm, and an obvious hysteresis loop appears in the range of medium relative pressure (P / P0=0.4~1.0), indicating that the catalyst has a mesoporous structure. As shown in Figure 6, the catalyst contains micropores, mesopores and macropores at the same time, with an average pore size of 4.1nm. At the same time, the specific surface area of ​​the catalyst reaches 252m 2 / g. The large specific surface area and wide pore size distribution of the catalyst greatly improve the accessibility between lignin macromolecules and the active centers of the catalyst, thereby enhancing mass transfer.

[0049] Figure 7 shows that the Co@NC-800 catalyst retains the regular dodecahedron morphology of the ZIF-67 precursor to a certain extent. As the pyrolysis proceeds, carbon nanotube structures appear on the catalyst surface. This is because the ZIF-67 undergoes reconstruction at high temperature. 2+ The metal is gradually reduced to Co, which catalyzes the graphitization of the ligand. As the degree of graphitization increases, carbon nanotubes form. A morphology is formed, with a hexadecahedron as the core and multiple carbon nanotubes extending from the surface. EDS mapping of the catalyst reveals that the Co is uniformly dispersed throughout the catalyst.

[0050] Figure 8 shows a high-intensity Co-N peak in the N 1s spectrum of the Co@NC-800 catalyst, indicating strong metal-support interactions. Peaks of graphitic N are also observed. This nitrogen-doped carbon structure can effectively modulate the catalyst's physical and chemical properties and electronic structure, thereby enhancing its catalytic performance.

[0051] Example 2: Preparation of zeolite imidazolate framework material derivative catalyst Co@NC-900

[0052] The preparation of the zeolite imidazolate framework material derivative catalyst Co@NC-900 includes the following two steps:

[0053] (1) Preparation of cobalt-based zeolite imidazolate framework material precursor ZIF-67: Weigh 1 g of 99% pure cobalt nitrate hexahydrate solid and 27.5 g of 99% pure 2-methylimidazole solid, dissolve them in 15 mL and 100 mL of deionized water, respectively, and stir for 5 min. The two solutions are mixed and stirred at room temperature for 24 h. The purple solid is separated by centrifugation, washed several times with deionized water and methanol, and then dried in a 60°C oven overnight. The purple powder ZIF-67 is obtained after grinding.

[0054] (2) Preparation of cobalt-based zeolite imidazolate framework material derivative catalyst Co@NC-900: The ZIF-67 prepared in (1) was placed in a tubular furnace and pyrolyzed at 900°C for 2 h under an inert gas atmosphere to obtain black powder Co@NC-900.

[0055] Example 3: Co@NC-800 catalytic depolymerization of bagasse lignin

[0056] (1) Lignin extraction: 10.0 g bagasse, 120 mL anhydrous ethanol, and 30 mL 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. Four times the volume of deionized water was added to the filtrate to precipitate the lignin. The mixture was allowed to stand for 12 h, filtered, dried, and ground to obtain bagasse lignin.

[0057] (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 replacing the gas in the reactor with argon five times, 1.0 MPa of argon was filled in. The temperature was raised 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 type, specifications: 30 m×0.53 mm×0.25 μm. Instrument heating program: from room temperature to 40°C, stabilized for 0.5 min, and maintained at 40°C for 13 min).

[0058] When measuring the molar amount of gas phase products, the system is at room temperature and pressure, and the mixed gas of hydrogen and internal standard argon can be regarded as an ideal gas mixture. According to the ideal gas state equation and the ideal gas partial volume law, the argon content in the gas phase product is n Ar (mmol) and hydrogen content n H2 (mmol) can be calculated according to formulas (1-1) to (1-2).

[0059] (1-1)

[0060] (1-2)

[0061] Among them, P Ar (Pa) is the pressure of argon gas filled into the reaction; V Ar (mL) is the volume of argon gas filled into the reaction; R is the gas constant, which is 8.314 J / (mol·K); T (K) is the room temperature. f is the correction factor, S H2is the hydrogen spectrum peak area; S Ar is the peak area of ​​the argon spectrum.

[0062] The reaction mixture was filtered, and the filter cake was washed with anhydrous ethanol. The filter cake was then soaked in 20 mL of tetrahydrofuran, filtered, washed, and dried. The recovered catalyst cake was then recycled. The liquid product and the anhydrous ethanol wash solution were transferred to a 25 mL volumetric flask. The internal standard, dimethyl phthalate, was added, and the volume was adjusted to 15 mL with anhydrous ethanol. 1.5 mL of the solution was aspirated from the volumetric flask, filtered through a 0.22 μm filter membrane, and transferred to an injection vial for qualitative and quantitative analysis 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 as follows: 50°C for 1 min, then increased to 250°C at a rate of 10°C / min and held for 31 min. The liquid products were qualitatively and quantitatively analyzed to determine the identity and yield of each product. 4 times the volume of deionized water was added to the remaining reaction solution, and the mixture was allowed to stand for 24 h to precipitate unreacted lignin, which was filtered and dried in a vacuum drying oven to constant weight to obtain brown solid regenerated lignin.

[0063] Table 1 Distribution and yield of volatile products obtained from lignin depolymerization

[0064]

[0065] The corresponding lignin conversion rate (C L ), the total yield of monophenol products (Y MP ), the yield of the main product 4-propyl-2,6-dimethoxyphenol (Y S2 ) and its selectivity (S S2 ) can be calculated according to 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 to the reaction; W MP (g) and W S2 (g) are the masses of monophenolic products and 4-propyl-2,6-dimethoxyphenol, respectively.

[0066] (1-3)

[0067] (1-4) (1-5)

[0068] (1-6)

[0069] The GC-FID spectrum of the product is shown in Figure 9, where the strongest peak signal is the main product. Combined with its mass spectrum (Figure 10), it can be confirmed that the main product is 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) > guaiacyl (G unit) > p-hydroxyphenyl (H unit).

[0070] Calculations show that the lignin conversion rate in this example 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.%, the selectivity was 46.0%, and the amount of hydrogen produced was 5.7 mmol.

[0071] Example 4: Depolymerization of Bagasse Lignin Using Co@NC-900 Catalyst

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

[0073] A reactor was charged with 0.1 g bagasse lignin, 0.125 g Co@NC-900 catalyst, and 10 mL isopropanol. After displacing the reactor atmosphere with argon five times, the reactor was filled with 1.0 MPa of argon and heated to 230°C at a rate of 5°C / min for 4 hours. After cooling to room temperature, the reaction gas was collected. The solid-liquid mixture was filtered, and the internal standard dimethyl phthalate was added to the liquid product. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

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

[0075] Example 5: Depolymerization of Bagasse Lignin Using Co@NC-800 Catalyst

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

[0077] A reactor was charged with 0.1 g of bagasse lignin, 0.1 g of Co@NC-800 catalyst, and 10 mL of isopropanol. After displacing the atmosphere with argon five times, the reactor was filled with 1.0 MPa of argon and heated to 230°C at a rate of 5°C / min for 4 hours. After cooling to room temperature, the reaction gases were collected and filtered to obtain a liquid product. The internal standard, dimethyl phthalate, was added to the liquid product. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

[0078] Calculations show that the bagasse lignin conversion rate in this example was 85.4%, the yield of monophenols 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: Depolymerization of Bagasse Lignin Using Co@NC-800 Catalyst

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

[0081] A reactor was charged with 0.1 g bagasse lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol. The atmosphere was replaced with argon five times, then filled with 1.0 MPa argon. The temperature was raised to 220°C at a rate of 5°C / min and held for 4 hours. After cooling to room temperature, the reaction gases were collected. The liquid product was filtered and added with dimethyl phthalate as an internal standard. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

[0082] Calculations show that the bagasse lignin conversion rate in this example was 89.4%, the yield of monophenols 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: Depolymerization of Bagasse Lignin Using Co@NC-800 Catalyst

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

[0085] A reactor was charged with 0.1 g bagasse lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol. After displacing the atmosphere with argon five times, the reactor was filled with 0.5 MPa argon and heated to 230°C at a rate of 5°C / min for 4 hours. After cooling to room temperature, the reaction gases were collected and filtered to obtain a liquid product. Dimethyl phthalate, an internal standard, was added to the liquid product. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

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

[0087] Example 8: Depolymerization of Bagasse Lignin Using Co@NC-800 Catalyst

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

[0089] A reactor was charged with 0.1 g bagasse lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol. The atmosphere was replaced with argon five times, then filled with 1.0 MPa argon. The temperature was raised to 230°C at a rate of 5°C / min and held for 2 hours. After cooling to room temperature, the reaction gas was collected and filtered to obtain a liquid product. Dimethyl phthalate, an internal standard, was added to the liquid product. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

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

[0091] Example 9: Depolymerization of Bagasse Lignin Using Co@NC-800 Catalyst

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

[0093] A reactor was charged with 0.1 g bagasse lignin, 0.125 g Co@NC-800 catalyst, and 10 mL anhydrous ethanol. After displacing the reactor atmosphere with argon five times, the reactor was filled with 1.0 MPa argon and heated to 230°C at a rate of 5°C / min for 4 hours. After cooling to room temperature, the reaction gases were collected and filtered to obtain a liquid product. The internal standard, dimethyl phthalate, was added to the liquid product. The gas and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

[0094] Calculations show that the bagasse lignin conversion rate in this example was 87.0%, the yield of monophenols was 15.0 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 5.4 wt.%, the selectivity was 36.0%, and 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) Lignin extraction: 10.0 g of bamboo, 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. Four times the volume of deionized water was added to the filtrate to precipitate the lignin. The mixture was allowed to stand for 12 h, filtered, dried, and ground to obtain bamboo lignin.

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

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

[0100] Example 11: Depolymerization of birch lignin using Co@NC-800 catalyst

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

[0102] (1) Lignin extraction: 10.0 g of birch, 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. Four volumes of deionized water were added to the filtrate to precipitate the lignin. The mixture was allowed to stand for 12 h, 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 a reactor. After replacing the gas in the reactor with argon five times, 1.0 MPa of argon was added. The temperature was raised 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 the internal standard dimethyl phthalate was added to the liquid product. The gas phase product and the liquid phase product were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

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

[0105] Example 12: Depolymerization of birch lignin using Co@NC-800 catalyst

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

[0107] (1) Lignin extraction: 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. Four times the volume of deionized water was added to the filtrate to precipitate the lignin. The mixture was allowed to stand for 12 h, filtered, dried, and ground to obtain poplar wood lignin.

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

[0109] Calculations show that the conversion rate of birch lignin in this example was 85.8%, the yield of monophenols was 16.0 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol 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] After the catalyst reacted in Example 3 was soaked in tetrahydrofuran for 12 hours, it was filtered and dried to constant weight, and the obtained catalyst solid was subjected to a circulation experiment. After the catalyst was recycled 4 times, the lignin conversion rate was 79.0%, the monophenol product yield was 12.8wt.%, and the 4-propyl-2,6-dimethoxyphenol yield was 3.6wt.%, the selectivity was 28%, and the amount of hydrogen produced was 5.4mmol. The reacted catalyst was directly circulated without treatment, and the catalyst activity was slightly decreased overall because unreacted substrate and oligomers were deposited on the catalyst surface, covering some active sites of the catalyst, resulting in a decrease in product yield.

[0112] Atomic absorption spectroscopy (AAS) was performed on the catalyst to determine the change in cobalt content before and after recycling. The AAS test was performed using a Hitachi Z-2300 flame atomic absorption spectrophotometer. The flame atomizer employed an air-acetylene flame with an atomization temperature range of 2100-2400°C. Before recycling, the cobalt content in the Co@NC-800 catalyst was 29 wt.%. After four cycles, the cobalt content in the catalyst was 28.7 wt.%. This indicates that the cobalt content remained nearly constant before and after recycling, indicating that there was virtually no cobalt loss in the catalyst. Combined with the X-ray energy spectrum test results of the catalyst, this indicates that the anchoring effect of nitrogen on Co in the catalyst effectively prevented Co loss during the catalytic process, thereby improving the stability of the catalyst.

[0113] As can be seen from the above embodiments, the present invention is a method for preparing monophenol chemicals by catalyzing the depolymerization of lignin using a zeolite imidazolate skeleton material derivative, which achieves selective depolymerization of lignin, with a lignin conversion rate of 79.8-89.4%, a monophenol product yield of 11.0-16.1wt.%, and a main product 4-propyl-2,6-dimethoxyphenol yield of 4.2-7.5wt.%, with a selectivity of 36.0-52.0%. At the same time, using isopropanol as the hydrogen supply solvent, after 0.1g of lignin is hydrogenolyzed, lignin consumes hydrogen in the reaction and hydrogenolyzes into monophenol products under the action of the catalyst. The amount of hydrogen produced per 10mL of isopropanol is 2.9-9.7mmol. In particular, the technology used in the present invention has the characteristics of renewable raw materials, simple reaction process, mild reaction conditions, and environmentally friendly catalysts, and can achieve intermittent or continuous reactions. The present invention has the above-mentioned technical effects, mainly having the following characteristics:

[0114] 1) Compared with the method of preparing catalysts by adding an external carbon source and a heteroatom doping source, loading an active metal, and finally reducing the catalyst, the catalyst of the present invention uses ZIF-67 with a highly ordered periodic network structure of coordination bonds as a precursor and prepares the Co@NC catalyst through a one-step pyrolysis. The interaction between the metal and the support is stronger, the dispersion of the active metal cobalt is higher, and the catalytic active site of cobalt is fully utilized, so that the catalyst can catalyze both solvent hydrogen production and lignin hydrogenolysis, improve the performance of catalyzing lignin hydrogenolysis, and achieve selective depolymerization of lignin.

[0115] 2) The catalyst preparation eliminates the need for high-purity hydrogen reduction. Instead, it utilizes the inherent reducing properties of carbon to enable thermal decomposition in an inert atmosphere, making it more economical and safer. Furthermore, the use of carbon in the precursor to reduce the active metal components enhances the interaction between the metal and the support, improving the activity and stability of the catalyst. After four cycles of the catalyst, no significant loss of cobalt metal was observed, demonstrating excellent catalytic stability.

[0116] 3) The catalyst preparation does not require external alkali to form pores. The multi-level porous catalyst can be obtained by utilizing the inherent 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 the catalyst.

[0117] Overall, the nitrogen-doped carbon structure of the catalyst can effectively regulate the catalyst's physical and chemical properties and electronic structure during lignin hydrogenolysis, improving its performance in lignin hydrogenolysis. Under environmentally friendly and mild conditions, the catalyst achieves higher selectivity for the main product, 4-propyl-2,6-dimethoxyphenol, ranging from 36.0% to 52.0%. Furthermore, the catalyst exhibits excellent substrate compatibility with lignin from diverse biomass sources, demonstrating efficient catalytic hydrogenolysis of lignin from sources such as bagasse, bamboo, poplar, and birch.

[0118] The implementation methods of the present invention are not limited to the above-mentioned embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principles of the present invention shall be considered as equivalent replacement methods and shall be included in the scope of protection of the present invention.

Claims

1. Method for preparing monophenolic chemicals by depolymerizing lignin with zeolitic imidazolate framework material derivatives as catalyst, characterized in that, Using lignin extracted from biomass as raw material, zeolitic imidazolate framework material derivative as catalyst, and organic small molecule alcohol as hydrogen-donating solvent, after replacement with inert gas, the pressure is increased to 0.5 - 3 MPa, the reaction temperature is controlled at 200 - 240 °C, and the reaction is stirred for 2 - 10 h to selectively depolymerize lignin into monophenolic chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol; the zeolitic imidazolate framework material derivative is obtained by pyrolyzing a cobalt-based zeolitic imidazolate framework material as a precursor under inert gas.

2. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using the zeolitic imidazolate framework material derivative according to claim 1, characterized in that, The precursor is prepared by the following method: Dissolve cobalt nitrate hexahydrate and 2-methylimidazole in deionized water respectively, mix them and stir at room temperature for 12 - 24 h, centrifuge, and dry to obtain the ZIF-67 precursor.

3. The method for preparing monophenolic chemicals by the depolymerization of lignin catalyzed by the zeolitic imidazolate framework material derivative according to claim 2, characterized in that, The molar ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 0.015 - 0.030:

1.

4. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using the zeolitic imidazolate framework material derivative according to claim 1, characterized in that, The inert gas is any one of nitrogen, argon, and helium.

5. The method for preparing monophenolic chemicals by depolymerizing lignin catalyzed by the zeolitic imidazolate framework material derivative according to claim 1, characterized in that, The pyrolysis refers to heating and decomposing the dried solid under an inert atmosphere, and the pyrolysis process is carried out in a tube furnace; the pyrolysis temperature is 600 - 900 °C, and the pyrolysis time is 1 - 4 h.

6. The method for preparing monophenolic chemicals by depolymerizing lignin with the zeolitic imidazolate framework material derivative according to claim 1, characterized in that The lignin extraction method is as follows: Place the dried biomass raw material and the extraction liquid in a hydrothermal reaction kettle, heat at 100 - 120 °C for 2 - 6 h, cool and filter, wash the solid phase with absolute ethanol, combine the washing liquid and the filtrate, add deionized water to precipitate the solid, stand for 12 - 24 h, filter and take the solid phase, and dry to obtain the lignin solid raw material; the biomass raw material is any one of bagasse, bamboo, corncob, poplar, pine, and birch, and after being crushed, it is sieved through 80 - 120 meshes.

7. The method for preparing monophenolic chemicals by depolymerizing lignin with the zeolitic imidazolate framework material derivative according to claim 6, characterized in that, The extraction liquid is a mixed liquid of absolute ethanol and dilute sulfuric acid, and their volume ratio is 2 - 5:1, and the concentration of the dilute sulfuric acid is 0.2 - 0.5 M; the dosage of the extraction liquid per gram of biomass raw material is 10 - 20 mL.

8. The method for preparing monophenolic chemicals by depolymerizing lignin with the zeolitic imidazolate framework material derivative according to claim 1, wherein The organic small molecule alcohol is any one of methanol, ethanol, ethylene glycol, isopropyl alcohol, and butanol.

9. The method for preparing monophenolic chemicals by depolymerizing lignin with the zeolitic imidazolate framework material derivative according to claim 1, characterized in that, The mass ratio of the zeolitic imidazolate framework material derivative catalyst to lignin is 0.5 - 1.5:

1.

10. The method for preparing monophenolic chemicals by depolymerizing lignin with the zeolitic imidazolate framework material derivative according to claim 1, characterized in that, 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 4-hydroxybenzoate.

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