Metal catalyst for oxidative depolymerization of industrial lignin as well as preparation method and application of metal catalyst

Abstract

The invention belongs to the technical field of catalysts, and particularly relates to a metal catalyst for oxidative depolymerization of industrial lignin as well as a preparation method and application of the metal catalyst. By controlling synthesis conditions, the alpha-Fe2O3 carrier with a small particle size and a high defect state is obtained, the specific surface area of the catalyst and the exposed number of surface active sites are effectively increased, and the reaction activity and reaction rate of the catalyst are remarkably enhanced. And secondly, Co / alpha-Fe2O3-D, Mn / alpha-Fe2O3-D and Cr / alpha-Fe2O3-D catalysts are prepared by adopting an atomic layer deposition method, so that the metal dispersibility and the interface synergistic effect are effectively improved, the catalysts are endowed with excellent structural stability and sulfur poisoning resistance, and the catalyst is suitable for desulfurization and efficient catalytic conversion of industrial lignin under harsh reaction conditions such as high temperature and high pressure. Experimental results show that after 8 hours of reaction, the depolymerization rate of lignin can reach 60%, the conversion rate of high-added-value products of aromatic aldehyde and aromatic acid exceeds 40%, and the overall performance is obviously superior to the existing similar technical level.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, specifically relating to a metal catalyst for industrial lignin oxidative depolymerization, its preparation method, and its application. Background Technology

[0002] Lignin, the second largest renewable resource after cellulose, is the only natural polymer containing a benzene ring. Globally, approximately 70 million tons of lignin are produced annually as a byproduct of the pulp and paper industry. However, only 2%–5% of industrial lignin (mainly lignin sulfonates) is used as an additive in building materials. The majority of industrial lignin is burned or dumped as low-value fuel, resulting not only in resource waste but also severe environmental pollution.

[0003] Currently, lignin, with its renewable nature, rich functional group structure, and excellent UV shielding and antioxidant functions, has been widely used in the modification of polymer materials such as PVA, significantly improving the mechanical properties, thermal stability, water vapor barrier properties, and biodegradability of these materials, thus promoting the development of high-value-added composite materials. Simultaneously, based on lignin's self-assembly behavior and silver ion adsorption characteristics, novel composite films and hydrogels with antibacterial and conductive properties have been constructed, expanding its applications in packaging, medical, and electronic fields. However, lignin still faces challenges in its applications, including poor dispersibility, weak interfacial compatibility, low purity of industrial lignin, limited reactivity, and complex and costly modification processes in some areas, which restrict its further development in large-scale, high-performance, and functional applications.

[0004] Therefore, there is an urgent need to promote the full depolymerization of lignin through technological means, realize its high value through fine chemical pathways, improve resource management, and promote sustainable development. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems existing in the conventional technology and to provide a metal catalyst for industrial lignin oxidative depolymerization, its preparation method and application.

[0006] To achieve the above-mentioned technical objectives and effects, the present invention is implemented through the following technical solution:

[0007] This invention provides a method for preparing a metal catalyst, comprising the following steps:

[0008] S1 and α-Fe2O3 were repeatedly washed with deionized water and anhydrous ethanol, centrifuged, and then dried in an oven at 80–120°C for 12–24 h. Finally, the dried precursor was calcined in a hydrogen atmosphere at 250–350°C with a heating rate of 5°C / min for 2–4 h to obtain a high-defect α-Fe2O3 support, denoted as α-Fe2O3-D.

[0009] S2. Using atomic layer deposition (ALD), Co / Mn / Cr metals are loaded onto the support obtained in step S1 to form Co / α-Fe₂O₃-D, Mn / α-Fe₂O₃-D, or Cr / α-Fe₂O₃-D catalysts. Ace-Co / Mn / Cr is used as a precursor to deposit Co / Mn / Cr single atoms on the α-Fe₂O₃-D surface. A carrier gas composed of high-purity reducing gas is introduced, and the reaction process is carried out within 10... -1 ~10 -3 The test was conducted under a vacuum of Pa.

[0010] S3. Place Ace-Co / Mn / Cr in a precursor container and heat to 120–160°C. Maintain the reaction chamber temperature at 240–300°C and the delivery pipeline temperature at 120–180°C. During the ALD process of Co / Mn / Cr, Ace-Co / Mn / Cr vapor is exposed to the α-Fe2O3-D substrate surface for deposition using a circulating method. Place the α-Fe2O3-D substrate in a reactor and expose it to Ace-Co / Mn / Cr vapor multiple times at 280°C for 6–10 minutes each time to ensure complete reaction between the precursor and the substrate surface. Finally, Co / α-Fe2O3-D, Mn / α-Fe2O3-D, or Cr / α-Fe2O3-D catalysts are obtained.

[0011] Furthermore, the lignin nanoparticle raw material is sulfonated lignin with a sulfur content of 0.01% to 8% and a β-O-4 bond retention rate of <70%.

[0012] Furthermore, the catalyst support was α-Fe₂O₃-D with a particle size of 50–100 nm and a surface oxygen vacancy content of 30–40%. Positron annihilation spectroscopy results showed that the t² lifetime of the prepared α-Fe₂O₃-D material increased from 235 ps to 310 ps, ​​and the I₂ intensity accounted for 37%. Combined with the increase in S-parameters and SW joint analysis, it was confirmed that a large number of surface oxygen vacancy defects were enriched. Simultaneously, the O₁s spectrum showed a significant increase in the area ratio of the defect oxygen peak at 531.2 eV, and the Fe 2p spectrum showed a significant increase in Fe... 2+ Signal enhancement.

[0013] Furthermore, highly dispersed single-atom Co / Mn / Cr atoms are stably anchored in the oxygen vacancies or defect regions of α-Fe₂O₃-D, forming a stable metal-oxygen-iron (MO-Fe) synergistic structure, promoting interfacial reactions, electron transfer, and COC bond activation. Positron annihilation spectroscopy results show that after Co single-atom deposition, the t₂ lifetime in the material further increases from 310 ps to 355 ps, the I₂ intensity ratio increases by 30-50%, and the S-parameter increases simultaneously; after Mn single-atom deposition, the t₂ lifetime in the material further increases from 310 ps to 370 ps, ​​the I₂ intensity ratio increases to 30-50%, and the S-parameter increases simultaneously; after Cr single-atom deposition, the t₂ lifetime in the material is between 355-370 ps, ​​the I₂ intensity ratio increases to 30-50%, and the S-parameter increases simultaneously. SW joint analysis indicates that the surface defect structure is further enriched and reconstructed during single-atom deposition.

[0014] Furthermore, the catalyst is a Co / α-Fe2O3-D, Mn / α-Fe2O3-D, or Cr / α-Fe2O3-D catalyst with a metal content of 5-10 wt%. Spectroscopic results show that Fe in the original α-Fe₂O₃-D is mainly in the high oxidation state. 3+ It exists in this form. After single-atom Co / Mn / Cr deposition, new Fe appears in the spectrum. 2+ The signal, accompanied by a significant increase in the quadrupole splitting parameter (ΔE_Q), indicates an enhanced asymmetry in the local electric field. Fitting results show that Fe... 2+ The proportion increased from nearly 0% to approximately 25%, while Fe 3+ The proportion decreased accordingly. Combined with positron annihilation spectroscopy and XPS results, this further proves that defect enrichment and interface electronic rearrangement induced some Fe... 3+ Reduced to Fe 2+ This reveals a synergistic regulatory mechanism between interface structure and electronic state during single-atom deposition. Furthermore, EXAFS results show that it exists in a five-coordinate (FeO5) form. EXAFS and XANES results indicate the presence of Co. 2+ Co 3+ Both forms have an average coordination number of 4–5. EXAFS and XANES results show the presence of Mn. 2+ Mn 3+ Both forms have an average coordination number of 4–5. EXAFS and XANES results show the presence of Cr. 3+ One form has an average coordination number of 6.

[0015] Furthermore, the reducing gases include hydrogen (H2), ammonia (NH3), methane (CH4), or carbon monoxide (CO), forming a stable nitrogen-reducing atmosphere mixture environment, which is used to regulate the surface defect structure of materials and the metal anchoring process.

[0016] The present invention also provides a metal catalyst for industrial lignin oxidative depolymerization, which is prepared by the above-described preparation method.

[0017] This invention also provides the application of a metal catalyst in the industrial oxidative degradation of lignin into aromatic monomers.

[0018] The beneficial effects of this invention are:

[0019] 1. This invention proposes a method for preparing and modifying a metal catalyst for the oxidative depolymerization of industrial lignin. By controllably constructing a high-defect α-Fe2O3-D support and employing precise atomic layer deposition technology, the desulfurization and efficient catalytic conversion of industrial lignin under harsh reaction conditions such as high temperature and high pressure are achieved.

[0020] 2. The α-Fe2O3-D support prepared by this invention has good crystal form, controllable particle size, and abundant surface defects, which significantly improves the specific surface area, active site exposure and interface regulation ability, providing an excellent structural basis for constructing efficient and stable heterogeneous catalytic systems.

[0021] 3. This invention prepares Co / α-Fe2O3-D, Mn / α-Fe2O3-D, and Cr / α-Fe2O3-D catalysts using a modified atomic layer deposition method. This method can control the deposition amount at the single-layer or even single-atom level, precisely adjust the content and dispersion of the supported metal, avoid agglomeration and particle growth in traditional wet processes, and significantly improve the metal utilization rate and active site exposure of the catalyst.

[0022] Of course, any product implementing this invention does not necessarily need to achieve all of the above advantages at the same time. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 The image shows the XRD pattern of the α-Fe2O3-D supported metal catalyst.

[0025] Figure 2 Scanning electron microscope image of the α-Fe2O3-D supported metal catalyst;

[0026] Figure 3 Data on the depolymerization activity of α-Fe2O3-D catalysis in the industrial lignin production process. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] This invention discloses a method for preparing a metal catalyst for the oxidative depolymerization of industrial lignin. By controlling the synthesis conditions, a small-particle-size, high-defect-state α-Fe₂O₃ (α-Fe₂O₃-D) support is obtained. This effectively increases the specific surface area and the number of exposed surface active sites of the catalyst, significantly enhancing its reactivity and reaction rate. Furthermore, Co / α-Fe₂O₃-D, Mn / α-Fe₂O₃-D, and Cr / α-Fe₂O₃-D catalysts are prepared using atomic layer deposition (ALD), effectively improving metal dispersion and interfacial synergy, endowing the catalysts with excellent structural stability and resistance to sulfur poisoning. These catalysts are suitable for the desulfurization and efficient catalytic conversion of industrial lignin under harsh reaction conditions such as high temperature and high pressure. Experimental results show that after 8 hours of reaction, the lignin depolymerization rate can reach 60%, and the conversion rate of high-value-added products such as aromatic aldehydes and aromatic acids exceeds 40%, with overall performance significantly superior to existing similar technologies.

[0029] The specific embodiments of the present invention are as follows:

[0030] Example 1

[0031] The α-Fe₂O₃-D support was prepared using the following method:

[0032] 1 g of α-Fe₂O₃ precursor was weighed and added to 30 mL of deionized water. The mixture was magnetically stirred for 30 min at room temperature to ensure thorough dispersion. The suspension was then sonicated for 20 min, washed three times with deionized water and anhydrous ethanol, and centrifuged. It was then dried in air at 80 °C for 12 h. Finally, the dried precursor was placed in a tube furnace and calcined at 300 °C with a heating rate of 5 °C / min for 2 h under a hydrogen atmosphere to obtain α-Fe₂O₃-D solid.

[0033] Example 2

[0034] The preparation methods for Co / α-Fe₂O₃-D, Mn / α-Fe₂O₃-D, and Cr / α-Fe₂O₃-D catalysts are as follows:

[0035] Using α-Fe₂O₃-D from Example 1, along with cobalt acetylacetone, manganese acetylacetone, and chromium acetylacetone (Ace-Co / Mn / Cr), as precursors, Co / Mn / Cr single atoms were deposited on the α-Fe₂O₃-D support. The carrier gas consisted of high-purity hydrogen (H₂), ammonia (NH₃), methane (CH₄), or carbon monoxide (CO), and the reaction was carried out under a vacuum controlled at 10 °C. -1 Up to 10 -3 The reaction was carried out under the following conditions: Ace-Co / Mn / Cr was placed in a precursor container and heated to 140°C. The reaction chamber temperature of the reactor was maintained at 280°C, and the delivery pipeline temperature was maintained at 150°C. During the ALD process of Co / Mn / Cr, Ace-Co / Mn / Cr vapor was circulated and exposed to the surface of the α-Fe₂O₃-D substrate for deposition. Approximately 0.2 g of α-Fe₂O₃-D substrate was placed in the reaction chamber and repeatedly exposed to Ace-Co / Mn / Cr vapor at 280°C for 6 minutes each time to ensure complete reaction between the precursor and the substrate surface. Finally, Co / α-Fe₂O₃-D, Mn / α-Fe₂O₃-D, and Cr / α-Fe₂O₃-D catalysts were obtained.

[0036] Example 3

[0037] Industrial lignin oxidation degradation performance tests of Co / α-Fe2O3-D, Mn / α-Fe2O3-D, and Cr / α-Fe2O3-D catalysts:

[0038] Taking the Co / α-Fe₂O₃-D catalyst as an example, 1g of lignin nanoparticles, 0.5g of α-Fe₂O₃-D supported Co catalyst, 0.1g of Fe powder, 8mL of 30wt% H₂O₂, 10mL of n-hexane, and 10mL of deionized water were sequentially added to a 50mL high-pressure reactor. After sealing the reactor, oxygen was introduced to replace the air in the reactor three times to eliminate air interference. The stirring device was turned on, and the stirring speed was set to 600r / min to ensure thorough mixing of the reactants. The reactor was heated to 110℃ and the pressure was increased to 2MPa, and the reaction was maintained at this temperature for 5h. Then, the temperature was increased to 150℃ and the pressure was increased to 4MPa, and the reaction was carried out for 3h. After the reaction was completed, the reactor was naturally cooled to room temperature, and the catalyst and unreacted lignin nanoparticle residue were separated by filtration. The filtrate was extracted three times with ethyl acetate, and the extracts were combined. The ethyl acetate was removed by vacuum distillation to obtain the depolymerization product.

[0039] Product analysis: Liquid chromatography (LC) was used to assess the change in the relative molecular mass of lignin before and after depolymerization. A variety of small molecule aromatic compounds were detected, such as vanillin, vanillic acid, syringaldehyde, syringic acid, p-hydroxybenzaldehyde, and p-hydroxybenzoic acid.

[0040] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a metal catalyst, characterized in that, Includes the following steps: S1 and α-Fe2O3 were repeatedly washed with deionized water and anhydrous ethanol, centrifuged, and then dried in an oven at 80–120°C for 12–24 h. Finally, the dried precursor was calcined in a hydrogen atmosphere at 250–350°C with a heating rate of 5°C / min for 2–4 h to obtain a high-defect α-Fe2O3 support, denoted as α-Fe2O3-D. S2. Using atomic layer deposition (ALD), Co / Mn / Cr metals are loaded onto the support obtained in step S1 to form Co / α-Fe₂O₃-D, Mn / α-Fe₂O₃-D, or Cr / α-Fe₂O₃-D catalysts. Ace-Co / Mn / Cr is used as a precursor to deposit Co / Mn / Cr single atoms on the α-Fe₂O₃-D surface. A carrier gas composed of high-purity reducing gas is introduced, and the reaction process is carried out within 10... -1 ~10 -3 The test was conducted under a vacuum of Pa. S3. Place Ace-Co / Mn / Cr in a precursor container and heat to 120–160°C. Maintain the reaction chamber temperature at 240–300°C and the delivery pipeline temperature at 120–180°C. During the ALD process of Co / Mn / Cr, Ace-Co / Mn / Cr vapor is exposed to the α-Fe2O3-D substrate surface for deposition using a circulating method. Place the α-Fe2O3-D substrate in a reactor and expose it to Ace-Co / Mn / Cr vapor multiple times at 280°C for 6–10 minutes each time to ensure complete reaction between the precursor and the substrate surface. Finally, Co / α-Fe2O3-D, Mn / α-Fe2O3-D, or Cr / α-Fe2O3-D catalysts are obtained.

2. The preparation method according to claim 1, characterized in that, The raw material for lignin nanoparticles is sulfonated lignin with a sulfur content of 0.01% to 8% and a β-O-4 bond retention rate of <70%.

3. The preparation method according to claim 1, characterized in that, The catalyst support is α-Fe2O3-D with a particle size of 50-100 nm and a surface oxygen vacancy content of 30-40%.

4. The preparation method according to claim 1, characterized in that, Highly dispersed single-atom Co / Mn / Cr atoms are stably anchored in the oxygen vacancies or defect regions of α-Fe2O3-D, forming a stable metal-oxygen-iron synergistic structure, which promotes interfacial reactions, electron transfer and COC bond activation.

5. The preparation method according to claim 1, characterized in that, The catalyst is a Co / α-Fe2O3-D, Mn / α-Fe2O3-D or Cr / α-Fe2O3-D catalyst with a metal content of 5-10 wt%.

6. The preparation method according to claim 1, characterized in that, Reducing gases include hydrogen, ammonia, methane, or carbon monoxide, forming a stable nitrogen-reducing atmosphere mixture environment, which is used to control the surface defect structure of materials and the metal anchoring process.

7. A metal catalyst for the industrial oxidative depolymerization of lignin, characterized in that, It is prepared by the preparation method according to any one of claims 1-6.

8. The application of the metal catalyst according to claim 7 in the industrial oxidative degradation of lignin into aromatic monomers.

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