A rhenium-tungsten nano-alloy cluster catalyst and its catalytic application in lignin depolymerization to prepare phenolic compounds

Abstract

The application discloses a rhenium-tungsten nano-alloy cluster catalyst and catalytic application thereof in preparation of phenolic compounds through lignin depolymerization. The catalyst is composed of a mesoporous Al2O3 carrier and tungsten core-rhenium shell structure nano-alloy clusters loaded thereon. Diffusion and deposition growth behaviors of rhenium atoms at the junction of tungsten nano-cluster crystal faces (110) are simulated through molecular dynamics, and a structure with high active site enrichment is formed by controlling temperature and other factors. The preparation process comprises the steps of carrier pretreatment, active component loading, calcination and activation. In application, the application controls temperature in sections, preferentially breaks lignin beta-O-4 bonds and promotes the generation of monophenol, and combines with MgO to regulate the acidity and alkalinity to inhibit excessive hydrogenation. In the rhenium-tungsten nano-alloy cluster constructed, tungsten activates C-O bonds with strong electron transfer capacity to promote directional rupture, solving the problem of low depolymerization efficiency; and rhenium stabilizes intermediates with high hydrogenation activity, avoids side reactions and realizes directional generation of monophenol.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanocatalytic materials, in particular to a rhenium-tungsten nanometer alloy cluster catalyst and its catalytic application in the preparation of phenolic compounds by lignin depolymerization. BACKGROUND

[0002] Traditional lignin depolymerization technology faces insurmountable bottlenecks in industrial application. On the one hand, the depolymerization efficiency has been at a low level for a long time, and the conversion rate is generally less than 40%. This is due to the fact that lignin is a complex aromatic polymer compound, and there are a large number of stable chemical bonds in its molecular structure, especially C-O bonds (of which the β-O-4 bond accounts for the highest proportion, accounting for more than 50% of the total connecting bonds) and C-C bonds. These chemical bonds have very high bond energy and strong chemical stability, and are difficult to be effectively broken. Existing catalysts are limited by structural design and cannot accurately recognize and act on these specific chemical bonds, making it difficult to achieve selective breaking, resulting in the fact that most lignin raw materials cannot be fully decomposed, causing inefficient use of resources.

[0003] On the other hand, the system of depolymerization products is extremely complex, mainly mixed phenolic compounds, including guaiacol, syringol, p-ethylphenol and other components, and also accompanied by polycyclic aromatic hydrocarbons, naphthenes and other by-products, making the subsequent separation and purification process lengthy and costly.

[0004] The core of the above two problems is that when the chemical bonds in the lignin molecule are broken, the generated intermediates have high chemical activity, and without precise regulation, they are prone to random reactions such as excessive hydrogenation, rearrangement or polymerization, thereby generating a large number of non-target products, not only reducing the yield of target monophenolic compounds, but also greatly increasing the difficulty and economic cost of product separation.

[0005] Bimetallic nanoclusters exhibit excellent catalytic performance due to their unique electronic and geometric effects, and rhenium-tungsten alloys have been proven to have high catalytic activity for a variety of reactions (such as ethanol / glycerol conversion to olefins, synthesis of alpha-methylstyrene, etc.). However, existing bimetallic catalysts for lignin depolymerization have obvious deficiencies in structural regulation. Most preparation methods cannot achieve atomic-level ordered distribution of active components, resulting in insufficient exposure of active sites and weakening of synergistic effect, which cannot balance the bond breaking efficiency and product directionality.

[0006] Therefore, it is necessary to provide a rhenium-tungsten nanometer alloy cluster catalyst and its catalytic application in the preparation of phenolic compounds by lignin depolymerization to solve the above technical problems. SUMMARY

[0007] The present application aims to provide a rhenium-tungsten nano-alloy cluster catalyst and its catalytic application in lignin depolymerization to prepare phenolic compounds, so as to strengthen the breaking of beta-O-4 bond by using the strong electron transfer ability of tungsten, solve the problem of low traditional depolymerization efficiency, and reduce the generation of by-products by means of the hydrogenation activity of rhenium and the directionality of the core-shell structure, and solve the problem of complex products.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical solutions.

[0009] In the first aspect, the present application provides a rhenium-tungsten nano-alloy cluster catalyst, which is composed of a mesoporous Al2O3 carrier and tungsten core-rhenium shell structure nano-alloy clusters loaded thereon, wherein the atomic ratio of rhenium to tungsten is 1:3-1:5, and the catalyst is prepared by the following steps:

[0010] Step one: take the mesoporous Al2O3 carrier, grind and sieve, wash with deionized water and dry to obtain the pretreated mesoporous Al2O3 carrier for standby use;

[0011] Step two: add the tungsten core-rhenium shell structure nano-alloy clusters into anhydrous ethanol to form a uniform suspension by ultrasonic dispersion;

[0012] According to the loading ratio of the total mass of rhenium and tungsten accounting for 5%-10% of the mass of the mesoporous Al2O3 carrier, the pretreated mesoporous Al2O3 carrier is added to the above-mentioned suspension, and the tungsten core-rhenium shell structure nano-alloy clusters in the suspension are uniformly loaded on the surface of the carrier by stirring, and then the ethanol in the system is evaporated to obtain a solid precursor loaded with tungsten core-rhenium shell structure nano-alloy clusters;

[0013] The tungsten core-rhenium shell structure is formed by molecular dynamics simulation: a BCC Wulff structure tungsten nano-cluster substrate (containing 6 crystal faces (100) and 12 crystal faces (110)) is constructed by using the LAMMPS program, rhenium atoms are deposited at a rate of 1 atom / ns at 1600-2200K, and the diffusion of rhenium atoms at the junction of crystal faces (110) (diffusion depth 0.5-2 atomic layers) is regulated to make the proportion of tungsten atoms in the core area ≥80% and the proportion of rhenium atoms in the shell layer ≥70%, and finally form an alloy cluster containing body-centered cubic, face-centered cubic and hexagonal close-packed lattice, wherein the proportion of active sites at the junction of crystal faces (110) is ≥60%;

[0014] Step three: place the solid precursor obtained in step two in a tube furnace, heat to 280-320℃ in an air atmosphere and keep warm, remove residual impurities, and obtain a calcined product;

[0015] Step four: switch the air atmosphere in the tube furnace to a hydrogen atmosphere, heat to 420-480℃ and keep warm, and after cooling, introduce nitrogen protection to obtain an activated product;

[0016] Step five: the activated product obtained in step five is ground and sieved to obtain a rhenium-tungsten nano-alloy cluster catalyst.

[0017] Preferably, after the mesoporous Al2O3 carrier in step one is ground, it is washed 3-5 times with deionized water, the drying temperature is 100-120 DEG C, and the drying time is 10-14 hours.

[0018] In step two, the ultrasonic dispersion time is 20-40 minutes, the power is 250-350 W, the frequency is 35-45 kHz, the stirring rate is 250-350 r / min, the stirring time is 2-4 hours, and the temperature for evaporating ethanol is 70-90 DEG C.

[0019] In step two, the lattice ratio of the tungsten core-rhenium shell structure is controlled by temperature: when the deposition temperature is 2200 K, the body-centered cubic lattice ratio is greater than or equal to 70%, when the deposition temperature is 1800-2000 K, the hexagonal close-packed lattice ratio is greater than or equal to 20%, and after 1 ns relaxation treatment, the lattice distortion rate is less than or equal to 5%.

[0020] Preferably, in step three, the heating rate under an air atmosphere is 4-6 DEG C / min, and the holding time is 1.5-2.5 hours.

[0021] In step four, the flow rate of the hydrogen atmosphere is 40-60 mL / min, the heating rate is 1-3 DEG C / min, and the holding time is 2-4 hours.

[0022] In the activated catalyst, rhenium atoms are enriched at the step of the crystal face (110), and the synergistic effect with tungsten atoms reduces the C-O bond energy by 30%-40%.

[0023] Preferably, in step five, the activated product is ground and sieved to 80-120 mesh.

[0024] In the catalyst, the proportion of the crystal face (110) atoms to the total number of atoms in the cluster is greater than or equal to 50%, rhenium is mainly Re 4+ (60%-70%), tungsten is mainly W 6+ (75%-85%), and the synergistic effect of d-orbital electron transfer and hydrogen ion supply enhances the catalytic activity.

[0025] In a second aspect, the application provides a catalytic application of a rhenium-tungsten nano-alloy cluster catalyst in the preparation of phenolic compounds by lignin depolymerization, which comprises the following steps:

[0026] Step one: the lignin raw material is ground by ball milling to a particle size of 50-100 μm, and then sieved for standby use.

[0027] Step two: mix the lignin treated in step one with the catalyst in a mass ratio of 10:1, add a methanol-water mixed solvent to the mixed system, where the volume ratio of methanol to water is 3:1, and introduce 0.1%-0.5% of MgO powder based on the mass of the catalyst, stir until the lignin, catalyst and MgO are uniformly dispersed in the solvent, and then adjust the pH of the system to 3-4 with dilute sulfuric acid;

[0028] The MgO neutralizes part of the strong acid sites on the surface of the catalyst, that is, the high-activity acid sites at the center of the (100) crystal plane, thereby reducing the probability of excessive hydrogenation of monophenolic products at the active center and inhibiting the deep conversion of monophenols into non-target products.

[0029] Specifically, on the one hand, the MgO weakly interacts with the W 6+ sites exposed on the surface of the catalyst, reducing the adsorption energy of strong acid sites on monophenolic products from -5.2eV to -3.8eV and shortening the residence time of monophenol molecules at the active center; on the other hand, the basicity of MgO can buffer the pH fluctuations in the system, avoiding side reactions such as hydrogenation of the aromatic ring of monophenols (e.g., conversion of guaiacol to cyclohexanol) or dehydration of phenolic hydroxyl groups due to excessive local acidity. This site-selective neutralization and regulation reduces the probability of excessive hydrogenation of monophenolic products at the active center by more than 40%, inhibiting their deep conversion into non-target products such as naphthenes and polycyclic aromatic hydrocarbons at the molecular level, and significantly improving the directional retention rate of target products.

[0030] Step three: transfer the mixed system obtained in step two to a high-pressure reaction kettle, replace the air in the kettle with H2 after sealing, maintain a H2 pressure of 2-3 MPa, and use a stepwise temperature control method during the reaction:

[0031] First, raise the temperature to 210-230°C for 1.5-2.5h to preferentially break the β-O-4 bond in the lignin molecule, which is achieved by the high adsorption capacity of the active sites at the junction of the (110) crystal plane of the catalyst and the activation of the C-O bond by the transfer of d-orbital electrons of tungsten atoms; then raise the temperature to 230-250°C for 1.5-2.5h to promote the formation of monophenolic compounds, which is achieved by the combination of hydrogen protons provided by rhenium atoms and broken oxygen atoms to form hydroxyl groups, thereby directionally stabilizing monophenol intermediates. The entire reaction process is carried out at a stirring rate of 500-800 rpm; by controlling the temperature and time of the two stages, the excessive hydrogenation of monophenolic products can be reduced;

[0032] Specifically, the first temperature rising stage (210-230℃, 1.5-2.5h): the temperature range is precisely matched with the adsorption energy of active sites at the intersection of catalyst crystal face (110) and the energy barrier of β-O-4 bond breaking (about 360kJ / mol). The intersection of crystal face (110) forms an electron-rich microzone due to low atomic coordination number (6-8), which has strong coordination adsorption capacity (adsorption energy-4.5eV) for the oxygen atom of β-O-4 bond in lignin molecules, and can selectively wrap the two benzene ring units connected by C-O bond through steric hindrance effect; at the same time, the 5d orbitals of tungsten atom (W 6+ ) transfer to the 2p orbitals of oxygen atom, which makes the bond length of C-O bond from 1.43Å to 1.51Å, and the bond energy decreases by 30%-40%, which preferentially occurs homolysis under H2 atmosphere to generate intermediates containing phenoxy free radicals. The synergistic effect of spatial selective adsorption and electronic transfer activation in this stage makes the breaking efficiency of β-O-4 bond increase by more than 50% compared with traditional catalysts, and the inhibition rate of C-C bond breaking reaches 70%, realizing the directional deconstruction of lignin skeleton.

[0033] The second stage (230-250℃, 1.5-2.5h): after heating to this range, the 4f orbitals of rhenium atom (Re 4+ ) interact with the σ bond of H2, which efficiently dissociates and generates active hydrogen proton (H + ) through hydrogen overflow mechanism, which quickly combines with phenoxy radicals (containing ) generated in the first stage (reaction energy barrier is only 12kJ / mol) to form stable phenolic hydroxyl (-OH), which locks the active intermediate into monophenol structure; at the same time, the rhenium atom at the intersection of (110) crystal face combines with benzene ring through π-π stacking effect, which further inhibits the polymerization or rearrangement of intermediates. In this stage, the generation selectivity of monophenol products is improved to more than 80% by the dual action of hydrogen supply and steric hindrance stability, among which the directional generation of target products such as guaiacol and eugenol accounts for 60%-70%.

[0034] Step four: after the reaction is completed, the mixed system in the high-pressure reaction kettle is cooled to room temperature and the remaining pressure in the kettle is released, the catalyst is recovered from the reaction mixture by centrifugal separation, the remaining reaction liquid is distilled under reduced pressure to remove the methanol-water mixed solvent, the obtained crude product is purified by silica gel column chromatography, the target fraction containing monophenol compounds is collected and dried to obtain monophenol compounds.

[0035] As preferably, in step one, the lignin raw material is herbaceous plant straw or woody plant sawdust, and the zirconium oxide grinding ball is used for ball milling at a ball-to-material ratio of 10:1 for 2-4 hours, and the ball milling product is screened through a 150-300 mesh standard sieve; in step two, the stirring is magnetic stirring at 300-500 rpm for 30-60 minutes, and the dilute sulfuric acid has a concentration of 0.1-0.5 mol / L.

[0036] As preferably, in step three, the H2 purity is 99.99%, and the heating rate is 5-10 ℃ / min; the generated monophenolic intermediates at least include guaiacol, syringol and p-ethylphenol. The catalyst selectively adsorbs the β-O-4 structural unit through the active sites at the junction of the crystal face (110), inhibits excessive breaking of the C-C bond, and makes the monophenol selectivity ≥80%.

[0037] As preferably, in step three, the lignin conversion rate is 65%-75%, and the monophenolic compound selectivity is ≥80%; in the monophenolic compound, guaiacol accounts for 30%-40%, and syringol accounts for 25%-35%.

[0038] As preferably, in step four, the centrifugal separation speed is 8000-10000 rpm for 10-15 min; the vacuum degree of the reduced pressure distillation is 0.08-0.1 MPa, and the temperature is 60-80 ℃, and a rotary evaporator is used at a rotation speed of 50-100 rpm; the silica gel column chromatography uses silica gel with a particle size of 100-200 mesh, and the ratio of the column diameter to the column height is 1:8-1:12, and the eluent is petroleum ether-ethyl acetate with a volume ratio of 5:1; the drying is vacuum drying at 40-60 ℃ for 12-24 hours, and the monophenolic compound with a purity ≥95% is obtained.

[0039] As preferably, the catalytic application of the rhenium-tungsten nano-alloy cluster catalyst further includes a catalyst regeneration step: the centrifugally recovered catalyst is first ultrasonically washed with 5-10 times the mass of anhydrous ethanol at a power of 300 W for 30 minutes, then soaked in a 0.05 mol / L H2SO4 solution for 1 hour to dissolve the residues, and finally washed with deionized water until neutral; after being dried at 120 ℃ for 2-4 hours, the catalyst is secondarily activated at 400 ℃ under a H2 atmosphere for 2 hours to restore the active valence state of Re 4+ and W 6+ .

[0040] Compared with the prior art, the present application has the following beneficial effects:

[0041] 1. The present application uses the synergistic catalytic mechanism of the rhenium-tungsten nano-alloy cluster, and the strong electron transfer ability of tungsten can precisely act on the C-O bond in lignin, especially the β-O-4 bond, to weaken the chemical bond energy through d-orbital electron transfer, realize the directional breaking of stable chemical bonds, and fundamentally solve the problem that the traditional method is difficult to decompose due to high bond energy, thereby significantly improving the overall depolymerization efficiency of lignin.

[0042] 2. The present application can stabilize the intermediate after the C-O bond is broken by the high hydrogenation activity of rhenium, form a hydroxyl group by providing a hydrogen proton combined with an oxygen atom, avoid random reactions or excessive hydrogenation of the intermediate, and guide the reaction to generate monophenolic compounds, thereby solving the defects of complex products and lack of directionality in traditional methods, and reducing the subsequent separation cost.

[0043] 3. Based on the atomic-level regulation of molecular dynamics simulation, the diffusion depth and distribution density of rhenium atoms can be controlled to avoid invalid wrapping of the rare metal rhenium and maximize its proportion participating in the catalytic reaction; the tungsten core-rhenium shell structure formed by regulation makes the active component rhenium enriched at the junction of the crystal face (110), thereby improving the exposure efficiency of the active site; at the same time, the directional regulation of the crystal lattice structure (body-centered cubic, hexagonal close-packed, etc.) reduces distortion, ensures catalytic performance, improves atomic economy, reduces resource waste, enhances the structural stability of the catalyst, prolongs the service life, and reduces the replacement frequency and cost in industrial application.

[0044] 4. In the catalytic application process of the rhenium-tungsten nanometer alloy cluster catalyst provided by the present application in the preparation of phenolic compounds from lignin depolymerization, the high-activity acid sites in the center of the catalyst crystal face (100) are neutralized by MgO, and weak interactions are formed with the surface W 6+ sites, reducing the adsorption energy of monophenolic products on strong acid sites and shortening their residence time in the active center. At the same time, the alkalinity of MgO can buffer the pH fluctuation of the system, avoid the local high acidity from causing side reactions such as hydrogenation of monophenol aromatic ring or dehydration of phenolic hydroxyl group, reduce the probability of excessive hydrogenation by more than 40%, and significantly improve the directional retention rate of the target product.

[0045] 5. In the catalytic application process of the rhenium-tungsten nanometer alloy cluster catalyst provided by the present application in the preparation of phenolic compounds from lignin depolymerization, through the segmented temperature control mechanism, in the first stage (210-230℃), the high adsorption capacity of the active sites at the junction of the catalyst crystal face (110) and the electron transfer effect of tungsten atoms are used to efficiently break the lignin β-O-4 bond, with a breakage efficiency improved by more than 50% compared to traditional catalysts, and a C-C bond breakage inhibition rate of 70%. In the second stage (230-250℃), rhenium atoms provide hydrogen protons to stabilize monophenol intermediates, and π-π stacking is used to inhibit polymerization or rearrangement, so that the selectivity of monophenol is improved to more than 80%, and the directional generation of target products accounts for 60%-70%. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 BCC Wulff and FCC Wulff structure schematic diagram, wherein Figure 1 a is a BCC Wulff structure, and the crystal face (110) and the crystal face (100) are marked;Figure 1 b is the FCC Wulff structure, marked with the crystal plane (111) and the crystal plane (100);

[0047] Figure 2 is the exchange process of rhenium atom and tungsten atom at a specific position and the corresponding energy barrier curve, wherein Figure 2 a- Figure 2 f is the exchange process of atoms at different positions (blue is rhenium atom and red is tungsten atom);

[0048] Figure 3 is Figure 2 the energy barrier curve of the exchange process (the horizontal axis is the number of images and the vertical axis is the energy);

[0049] Figure 4 is the diffusion diagram of rhenium atom on tungsten nanocluster at the initial stage of deposition, wherein Figure 4 a is the adsorption of rhenium atom and tungsten atom at the crystal plane (100) to form a sharp corner; Figure 4 b is the exchange of rhenium atom and surface atom of tungsten nanocluster; Figure 4 c is the adsorption of rhenium atom and tungsten atom at the crystal plane (110);

[0050] Figure 5 is the schematic diagram of vacancy formation and migration mechanism, wherein Figure 5 a is the initial state of adsorption of tungsten atom at the four-fold coordinated site of the crystal plane (100) and the existence of rhenium atom at the step; Figure 5 b- Figure 5 e is the vacancy formation by tungsten atom jump and the long chain jump process (blue is rhenium atom, red is tungsten atom and white is tungsten atom to be observed);

[0051] Figure 6 is the schematic diagram of tungsten atom migration mechanism under different conditions, wherein Figure 6 a- Figure 6 d is the schematic diagram of four different conditions (with or without adsorbed atom and rhenium atom);

[0052] Figure 7 is Figure 6 the corresponding energy barrier curve (the horizontal axis is the number of images and the vertical axis is the energy);

[0053] Figure 8 is the surface diffusion diagram of rhenium-tungsten nanocluster under different sizes (rhenium-tungsten ratio 1:1, temperature 2000K), Figure 8 a is the cross-sectional diagram of total atom number 118; Figure 8 b is the cross-sectional diagram of total atom number 678; Figure 8 c is the cross-sectional diagram of total atom number 2042;

[0054] Figure 9 is the diagram of the number of atoms of each lattice type varying with the number of deposited Re atoms,Figure 9 a represents the change in the number of BCC lattice atoms as a function of the number of deposited rhenium atoms; Figure 9 b represents the change in the number of FCC lattice atoms as a function of the number of deposited rhenium atoms; Figure 9 c represents the variation of the number of HCP lattice atoms with the number of deposited rhenium atoms; Figure 9 d represents the curve showing the change in the number of tungsten and rhenium atoms in the FCC lattice;

[0055] Figure 10 Line graphs showing the lignin conversion rate in the catalytic application of lignin depolymerization in the preparation of phenolic compounds in Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0056] Figure 11 Line graphs showing the monophenol selectivity in the catalytic application of lignin depolymerization to prepare phenolic compounds in Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0057] Figure 12 Line graphs showing the relative efficiency of β-O-4 bond breaking in the catalytic application of lignin depolymerization in the preparation of phenolic compounds in Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0058] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. 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.

[0059] In this invention, crystal planes (100), (110), and (111) are all defined based on the Miller indices in crystallography and are used to characterize the atomic arrangement planes with different orientations in a crystal.

[0060] Crystal plane (100): The Miller index is (h,k,l) ​​= (1,0,0), which refers to the atomic plane that is parallel to the y-axis and z-axis of the crystal coordinate system and intersects only the x-axis;

[0061] Crystal plane (110): The Miller index is (h,k,l) ​​= (1,1,0), which refers to the atomic plane that is parallel to the z-axis of the crystal coordinate system and intersects both the x-axis and y-axis;

[0062] Crystal plane (111): The Miller index is (h,k,l) ​​= (1,1,1), which refers to the atomic plane that intersects the x-axis, y-axis and z-axis of the crystal coordinate system.

[0063] Regarding the crystal structures involved in this invention (such as body-centered cubic (BCC) and face-centered cubic (FCC), the differences in atomic arrangement among the three are as follows:

[0064] 1. Crystal plane (100):

[0065] like Figure 1 As shown in Figure a, in the BCC structure tungsten nanoclusters of this invention, the crystal plane (100) is a cubic 'face'-shaped plane with a low surface atomic density and an atomic coordination number of 8 (each surface atom is surrounded by 8 atoms). The center of this crystal plane contains highly active acidic sites, mainly composed of W... 6+ It is formed upon exposure and has a high adsorption energy (-5.2 eV) for monophenolic products.

[0066] 2. Crystal plane (110):

[0067] like Figure 1 As shown in Figure a, in the BCC structure tungsten nanoclusters, the crystal plane (110) is a 'faceted' plane connecting adjacent crystal planes (100), with a higher atomic density on the surface than on crystal plane (100), and an atomic coordination number of 6-8 (the atomic coordination number at the center of the crystal plane is higher than at the interface). Its significant feature is the presence of a large number of crystal plane interfaces, including the vertices where three crystal planes (110) intersect, the edges where two crystal planes (110) intersect, and the intersection lines where crystal planes (110) and crystal plane (100) intersect. The atomic coordination number at these interfaces is 6, forming an electron-rich micro-region, which is a region where rhenium atoms are preferentially enriched (the distribution density is more than 50% higher than that at the center of crystal plane (100)).

[0068] 3. Crystal plane (111):

[0069] like Figure 1 As shown in b, in the FCC structural reference model, the crystal plane (111) is a 'triangular' plane with the highest surface atomic density and an atomic coordination number of 12, exhibiting strong chemical stability. In the BCC structure tungsten nanoclusters of this invention, the crystal plane (111) does not participate in the main catalytic reaction and exists only as a comparative structure.

[0070] This invention provides a method for regulating the configuration of rhenium-tungsten nano-alloy clusters to prepare rhenium-tungsten nano-alloy cluster catalysts, which serves as the theoretical basis for the preparation of rhenium-tungsten nano-alloy clusters, as follows:

[0071] 1. Model building and parameter setting: such as Figure 1 As shown, Figure 1 a demonstrates the BCC Wulff structure, Figure 1b demonstrates the FCC Wulff structure, in which the BCC Wulff structure clearly marks 6 crystal faces (100) and 12 crystal faces (110). This step uses the LAMMPS program to build a tungsten nanocluster model, adopts a BCC Wulff structure (containing 6 crystal faces (100) and 12 crystal faces (110)), and sets the initial number of atoms. The interaction between rhenium and tungsten atoms is simulated by the EAM potential function, and then the atoms in the system are tracked.

[0072] 2. Temperature equilibrium control: In the NVT ensemble, the Nose-Hoover method is used to maintain temperature equilibrium: first, the system is heated to the target temperature (such as 2000K) and maintained for 10ns, and then 200ps relaxation treatment is performed with a time step of 0.002ps.

[0073] 3. Rhenium atom deposition: In this step, rhenium atoms are randomly deposited on the surface of the tungsten nanocluster at a rate of 1 atom / ns, and the deposition process is visualized in real time by the OVITO tool. In the early stage of deposition, rhenium atoms preferentially adsorb on the crystal face (110) (because its area is larger than that of the crystal face (100)), and exchange with tungsten atoms at the intersection of the crystal face (110) - among them, the tungsten atoms at the intersection of three crystal faces (110) have the lowest exchange energy barrier and the highest exchange probability, followed by the tungsten atoms at the edge of two crystal faces (110). As shown in Figures 2-4 , Figure 2 Through Figure 2 a- Figure 2 f, the atomic exchange process diagram shows the exchange mechanism of rhenium atoms and tungsten atoms at different crystal face positions, combined with Figure 3 the corresponding energy barrier curve, especially the feature that the exchange energy barrier of the intersection of three crystal faces (110) is the lowest; Figure 4 then presents the adsorption and diffusion behavior of rhenium atoms on the crystal face (100) and the crystal face (110), verifying the rule that rhenium atoms preferentially adsorb on the crystal face (110).

[0074] 4. Vacancy formation and migration monitoring: During the deposition process, vacancies will form on the surface of the crystal face (100): when three tungsten atoms are adsorbed on the four-coordinate sites on the surface of the crystal face (100), the fourth site forms a vacancy, and then the adjacent tungsten atoms migrate in turn to fill the vacancy, forming a chain reaction. The presence of rhenium atoms can reduce the tungsten atom migration energy barrier (as low as 1.964eV), promoting vacancy migration. As shown in Figures 5-7 , Figure 5 a- Figure 5 e illustrates the formation of vacancies on the surface of the crystal face (100) and the chain reaction of tungsten atom migration to fill the vacancies, i.e. three tungsten atoms are adsorbed on four-coordinate sites to form a vacancy. Figure 6 is a schematic diagram of the tungsten atom migration mechanism under different conditions, Figure 6 a indicates that there are no adsorbed atoms and no Re atoms;Figure 6 b represents only Re atoms; Figure 6 c represents only adsorbed atoms; Figure 6 d represents both adsorbed atoms and Re atoms. Figure 7 The migration energy barriers of tungsten atoms with and without Re atoms are compared through the energy barrier curves, which shows the mechanism of Re atoms reducing the migration energy barrier of tungsten atoms. Figure 6 The migration energy barriers of tungsten atoms with and without Re atoms are compared through the energy barrier curves, which shows the mechanism of Re atoms reducing the migration energy barrier of tungsten atoms.

[0075] 5. Lattice structure analysis: The neighborhood analysis method is used to track the lattice transformation: when the number of deposited Re atoms is less than or equal to 800, the system is mainly in BCC lattice; when the number of Re atoms increases to more than 1200, the proportion of HCP and FCC lattices gradually increases. After depositing 1400 Re atoms, 1 ns relaxation treatment is carried out, and finally the number of atoms in BCC, FCC and HCP lattices is determined by statistics. As shown in Figure 9 a- Figure 9 a- Figure 9 d respectively show the change trend of the number of BCC, FCC and HCP lattice atoms with the amount of deposited Re atoms, especially the transition rule from BCC lattice as the main to the increase of HCP and FCC proportion.

[0076] 6. Size and temperature influence regulation: If the size effect needs to be studied, 58, 339, 641, 1021 atom tungsten nanoclusters can be constructed, the ratio of Re-tungsten is kept at 1:1, and the change of Re atom diffusion depth with size is observed (the larger the size, the more difficult it is for Re atoms to diffuse to the core). If the lattice type needs to be regulated, the deposition temperature can be adjusted (such as 2200K to promote BCC lattice, 1800-2000K to increase the proportion of HCP lattice). As shown in Figure 8 a- Figure 8 a- Figure 8 c are cross-sectional views of Re-tungsten nanoclusters with different total atom numbers (118, 678, 2042), which directly show the size effect that the larger the size, the more difficult it is for Re atoms to diffuse to the core, that is, the research method of constructing tungsten nanoclusters with different atom numbers in step 6.

[0077] In order to verify the catalytic application performance of the catalyst prepared by the above configuration regulation method of Re-tungsten nanometer alloy cluster in the preparation of phenolic compounds from lignin depolymerization, the present application further provides the following examples:

[0078] Example 1:

[0079] Configuration regulation and preparation steps of Re-tungsten nanometer alloy cluster catalyst:

[0080] A BCC Wulff structure tungsten nanocluster (initial number of atoms 1021, containing 6 crystal faces (100) and 12 crystal faces (110)) was constructed by LAMMPS program, and rhenium atoms (total amount 255, rhenium-tungsten atomic ratio 1:4) were deposited at a rate of 1 atom / ns at 2200K. The temperature was balanced in the NVT system by the Nose-Hoover method (heated to 2200K for 10ns, 0.002ps step relaxation for 200ps), and the rhenium atoms were preferentially enriched at the crystal face (110) step by taking advantage of the low energy barrier at the crystal face (110) junction (0.3eV lower than the center of the crystal face (100)), and the diffusion depth was controlled to be 1 atomic layer. Finally, a tungsten core-rhenium shell structure (tungsten core accounted for 83%, rhenium shell accounted for 75%) was formed, the body-centered cubic (BCC) lattice accounted for 72%, the active site at the crystal face (110) junction accounted for 66%, and the lattice distortion rate was 3.5% after 1ns relaxation.

[0081] Example 1 uses a hydrothermal method to prepare rhenium-tungsten nanometer alloy clusters, and the specific steps are as follows:

[0082] Step one: according to the rhenium-tungsten atomic ratio 1:4, weigh the ammonium paratungstate and high-rhenium acid ammonium and dissolve them in deionized water, add 0.1mol / L citric acid as a chelating agent, the total molar ratio of citric acid to metal ions is 1:1, 60℃ magnetic stirring for 1 hour to form a homogeneous solution.

[0083] Step two: transfer the solution to a 50mL hydrothermal reactor, react at 180℃ for 12 hours, cool naturally for 10 minutes at 8000rpm, and collect the black precipitate.

[0084] Step three: wash the precipitate with deionized water and anhydrous ethanol alternately for 3 times, vacuum dry at 60℃ for 8 hours, and get tungsten-rhenium composite hydroxide precursor.

[0085] Step four: place the precursor in a tube furnace, heat to 800℃ at 5℃ / min under argon atmosphere, keep for 2 hours to decompose the hydroxide; switch to hydrogen atmosphere, flow rate 50mL / min, reduce at 450℃ for 3 hours, cool to get tungsten core-rhenium shell structure clusters.

[0086] Catalyst preparation steps:

[0087] Step one: grind the mesoporous Al2O3 carrier through a 200 mesh sieve, wash with deionized water for 3 times, dry at 110℃ for 12 hours to get a pretreated carrier;

[0088] Step two: add the above core-shell structure cluster to anhydrous ethanol, ultrasonic dispersion for 30 minutes at 300W, ultrasonic frequency 40kHz, add the pretreated carrier according to the total metal mass accounting for 8% of the carrier, stir at 300r / min for 3 hours, evaporate ethanol at 80℃ to get a solid precursor;

[0089] Step three: the precursor is heated to 300℃ at a rate of 5℃ / min in air atmosphere, and kept for 2 hours to completely remove residual organic matter and dispersant impurities;

[0090] Step four: switch to hydrogen atmosphere with a flow rate of 50mL / min, and heat to 450℃ at a rate of 2℃ / min and keep for 3 hours. After cooling, protect with nitrogen to obtain the activated product;

[0091] Step five: grind the activated product through a 100-mesh sieve to obtain the target catalyst, wherein Re 4+ accounting for 65%, W 6+ accounting for 80%.

[0092] Application steps of lignin depolymerization catalysis:

[0093] Step one: grind the lignin of herbaceous plant straw with zirconium oxide grinding balls at a ball-to-material ratio of 10:1 for 3 hours, and crush to 50-100μm and pass through a 200-mesh sieve;

[0094] Step two: mix the lignin with the catalyst at a mass ratio of 10:1, add a methanol-water mixed solvent with a volume ratio of 3:1, introduce 0.3% MgO powder based on the mass of the catalyst, and stir at 300rpm for 40 minutes until the system is uniformly dispersed. Adjust the pH to 3.5 with 0.1mol / L dilute sulfuric acid;

[0095] Step three: transfer the mixed system to a 100mL high-pressure reaction kettle, replace the air with 99.99%H2 for 3 times, maintain the pressure at 2.5MPa, and use staged temperature control: first heat to 220℃ at a rate of 5℃ / min, and keep for 2 hours; then heat to 240℃ at a rate of 5℃ / min, and keep for 2 hours, Re 4+ Stable monophenol intermediates are provided by hydrogen overflow. The stirring rate is 600rpm throughout the process;

[0096] Step four: after the reaction is completed, cool the high-pressure reaction kettle to room temperature and release the remaining pressure, and centrifuge at 8000rpm for 10 minutes to recover the catalyst. Remove the solvent by vacuum distillation of the reaction liquid at a vacuum degree of 0.09MPa and a temperature of 70℃. Purify the crude product by silica gel column chromatography with a column diameter to height ratio of 1:10 and petroleum ether-ethyl acetate eluent with a volume ratio of 5:1. After collecting the target fraction, vacuum dry at 50℃ for 18 hours to obtain monophenol compounds, wherein guaiacol accounts for 35% and eugenol accounts for 30%.

[0097] Example 2:

[0098] Configuration regulation and preparation steps of rhenium-tungsten nano-alloy cluster catalyst:

[0099] The tungsten nanocluster has 641 initial atoms, 160 rhenium atoms are deposited at 1800K (rhenium-tungsten ratio 1:4), and the proportion of hexagonal close-packed (HCP) lattice is increased to 22% by temperature control during deposition. The active site proportion at the intersection of the (110) crystal plane is 63%. The core area tungsten proportion in the core-shell structure is 81%, and the shell layer rhenium proportion is 72%. The lattice distortion rate is 4.2%.

[0100] Example 2 also uses a hydrothermal method to prepare rhenium-tungsten nanometer alloy cluster, which is different from the steps of example 1 only in that:

[0101] Step four: the precursor is placed in a tube furnace, heated to 700℃ at 5℃ / min under argon atmosphere, and kept for 2 hours; switch to hydrogen-argon mixed gas atmosphere, the volume ratio of hydrogen and argon is 1:1, the total flow rate is 60ml / min, reduce at 750℃ for 4 hours, and get the target cluster after cooling.

[0102] Catalyst preparation steps:

[0103] Step one: the mesoporous Al2O3 carrier is ground through a 200 mesh sieve, washed with deionized water for 3 times, dried at 110℃ for 12 hours, and the pretreated carrier is obtained;

[0104] Step two: the above core-shell structure cluster is added to anhydrous ethanol, ultrasonically dispersed for 30 minutes at 300W, the ultrasonic frequency is 40kHz, the pretreated carrier is added according to the proportion of 10% of the total metal mass of the carrier, stirred at 300r / min for 3 hours, and then evaporated at 80℃ to remove ethanol, and the solid precursor is obtained;

[0105] Step three: the precursor is heated to 320℃ at 5℃ / min in air atmosphere, and kept for 2 hours to completely remove residual organic matter and dispersant impurities;

[0106] Step four: switch to hydrogen gas atmosphere, the flow rate is 60ml / min, heat to 480℃ at 2℃ / min and keep for 3 hours, cool down and protect with nitrogen, and get the activated product;

[0107] Step five: the activated product is ground through a 100 mesh sieve, and the target catalyst is obtained, wherein Re 4+ The proportion is 63%, W 6+ The proportion is 82%.

[0108] Lignin depolymerization catalytic application steps:

[0109] Step one: the lignin of herbaceous plant straw is ground by zirconium oxide grinding ball, the ball to material ratio is 10:1, and the grinding time is 3 hours, and the powder is crushed to 50-100μm and passed through a 200 mesh sieve for use;

[0110] Step two: mixed with lignin and catalyst in the mass ratio of 10:1, added methanol-water mixed solvent in the volume ratio of 3:1, introduced 0.5% of MgO powder in the mass of catalyst, 300 rpm magnetic stirring for 40 minutes until the system was uniformly dispersed, and then adjusted the pH to 3.5 with 0.1 mol / L dilute sulfuric acid;

[0111] Step three: the mixed system was transferred to a 100 mL high-pressure reaction kettle, and after being sealed, 99.99% hydrogen was introduced to replace the air in the kettle for 3 times, and the hydrogen pressure was maintained at 2.5 MPa; the temperature was controlled in stages: first, the temperature was raised to 210℃ at a rate of 5℃ / min, and the reaction was maintained for 2.5 hours; then the temperature was raised to 230℃ at a rate of 5℃ / min, and the reaction was maintained for 2 hours, the whole process was stirred at a rate of 600 rpm;

[0112] Step four: after the reaction was completed, the high-pressure reaction kettle was cooled to room temperature and the remaining pressure was released, and the catalyst was recovered by centrifugation at 8000 rpm for 10 minutes; the reaction solution was distilled under reduced pressure to remove the solvent, the vacuum degree of the reduced pressure distillation was 0.09 MPa, and the temperature was 70℃; the crude product was purified by silica gel column chromatography with 100-200 mesh, the ratio of column diameter to height was 1:12, and the eluent was petroleum ether-ethyl acetate in the volume ratio of 5:1; after collecting the target fraction, it was dried at 50℃ under vacuum for 24 hours to obtain monophenolic compounds, in which guaiacol accounted for 33% and eugenol accounted for 28%.

[0113] Example 3:

[0114] Configuration regulation and preparation steps of rhenium-tungsten nano-alloy cluster catalyst:

[0115] The initial number of tungsten nanoclusters was 339 atoms, and 68 rhenium atoms were deposited at 2000K (rhenium-tungsten ratio 1:5), the active site ratio at the intersection of the (110) crystal face was 61%, the BCC lattice ratio was 68%, and the FCC lattice ratio was 15%, and the core-shell structure was complete (core tungsten 80%, shell rhenium 71%).

[0116] Example 3 also uses a hydrothermal method to prepare rhenium-tungsten nano-alloy clusters, and the difference from the steps of example 1 is only:

[0117] Step one: according to the rhenium-tungsten atomic ratio of 1:5, weigh the ammonium paratungstate and high-rhenium acid ammonium and dissolve them in deionized water, add 0.1 mol / L citric acid as a chelating agent, the total molar ratio of citric acid to metal ions is the same as in example 1, and the homogeneous solution is formed by magnetic stirring at 60℃ for 1 hour.

[0118] Step four: place the precursor in a tube furnace, heat to 800℃ at a rate of 5℃ / min under argon atmosphere, and maintain for 2 hours; switch to hydrogen atmosphere, flow rate 40 mL / min, reduce at 500℃ for 2 hours, and obtain the target cluster after cooling.

[0119] Catalyst preparation steps:

[0120] Step one: the mesoporous Al2O3 carrier is ground through a 200-mesh sieve, washed with deionized water for 3 times, and dried at 110°C for 12 hours to obtain a pretreated carrier;

[0121] Step two: the above core-shell structure cluster is added to anhydrous ethanol, ultrasonically dispersed for 30 minutes at a frequency of 40 kHz, and the pretreated carrier is added in a proportion of 5% of the total mass of the metal, stirred at 300 r / min for 3 hours, and then evaporated at 80°C to remove ethanol to obtain a solid precursor;

[0122] Step three: the precursor is heated to 280°C at a rate of 5°C / min in an air atmosphere, and kept for 2 hours to completely remove residual organic matter and dispersant impurities;

[0123] Step four: switch to a hydrogen atmosphere with a flow rate of 40 mL / min, heat to 420°C at a rate of 2°C / min, and keep for 2 hours, then cool down and protect with nitrogen to obtain an activated product;

[0124] Step five: the activated product is ground through a 100-mesh sieve to obtain the target catalyst, wherein Re 4+ 60%, W 6+ 78%.

[0125] Lignin depolymerization catalytic application steps:

[0126] Step one: the lignin of herbaceous plant straw is ground by a zirconium oxide grinding ball with a ball-to-material ratio of 10:1 for 3 hours, and is crushed to 50-100 μm and passed through a 200-mesh sieve for use;

[0127] Step two: mix the lignin with the catalyst in a mass ratio of 10:1, add a methanol-water mixed solvent with a volume ratio of 3:1, introduce 0.2% of MgO powder based on the mass of the catalyst, and stir at 300 rpm for 50 minutes until the system is uniformly dispersed, and then adjust the pH to 3.5 with 0.1 mol / L dilute sulfuric acid;

[0128] Step three: transfer the mixed system to a 100-mL high-pressure reaction kettle, replace the air in the kettle with 99.99% hydrogen for 3 times after sealing, maintain the hydrogen pressure at 2.5 MPa, and use a stepwise temperature control: first heat to 220°C at a rate of 5°C / min, keep for 2.5 hours, and then heat to 240°C at a rate of 5°C / min, keep for 2 hours, and Re 4+ Efficient dissociation of hydrogen gas to generate hydrogen protons, stable monophenol intermediates, and the whole process is stirred at a rate of 700 rpm;

[0129] Step four: After the reaction, the autoclave was cooled to room temperature and the residual pressure was released, and the catalyst was recovered by centrifugation at 8000 rpm for 10 minutes; the reaction solution was distilled under reduced pressure to remove the solvent, the vacuum degree was 0.09 MPa, and the temperature was 70°C; the crude product was purified by silica gel column chromatography (100-200 mesh), the ratio of column diameter to height was 1:8, and the eluent was petroleum ether-ethyl acetate (5:1, by volume); after the target fraction was collected, it was dried in a vacuum oven at 50°C for 12 hours to obtain a monophenolic compound, in which guaiacol accounted for 32% and eugenol accounted for 27%.

[0130] Comparative Example 1: Rhenium-tungsten mixed catalyst without core-shell structure

[0131] The difference between Comparative Example 1 and the examples is only that:

[0132] Catalyst preparation step: The rhenium-tungsten mixed catalyst was prepared by chemical co-precipitation method, but not a core-shell structure: sodium tungstate and ammonium perrhenate were dissolved in deionized water according to an atomic ratio of 1:4, and a mesoporous Al2O3 carrier (total metal mass accounting for 8% of the carrier) was added to obtain a rhenium-tungsten random mixed catalyst. The catalyst has no clear crystal face active site enrichment, Re 4+ accounts for 50%, and W 6+ accounts for 65%.

[0133] Comparative Example 1 has a significant decrease in lignin conversion rate due to the dispersion of active sites without a core-shell structure, a substantial decrease in β-O-4 bond breaking efficiency, and a significant decrease in monophenol selectivity.

[0134] Comparative Example 2: Single tungsten component catalyst

[0135] The difference between Comparative Example 2 and the examples is only that:

[0136] Catalyst preparation step: Only tungsten components were loaded, and a sodium tungstate solution was loaded on a mesoporous Al2O3 carrier (tungsten mass accounting for 8% of the carrier) to obtain a pure tungsten catalyst (without rhenium components).

[0137] Lignin depolymerization application step: the reaction was carried out according to the method of Example 1, and MgO was not introduced in step two.

[0138] Comparative Example 2 has a decrease in lignin conversion rate due to the lack of rhenium components, a substantial decrease in monophenol selectivity, and a significant increase in the proportion of by-products such as polycyclic aromatic hydrocarbons and naphthenes in the product.

[0139] Comparative Example 3: Single temperature reaction without segmented temperature control

[0140] The difference between Comparative Example 3 and the examples is only that:

[0141] Lignin depolymerization application step: the reaction temperature was a single 230°C, and the reaction lasted for 4 hours without segmented temperature control.

[0142] The comparative example 3 cannot realize the directional rupture of β-O-4 bond and the precise stabilization of monophenol intermediate, the lignin conversion rate is reduced, the monophenol selectivity is obviously decreased, and the proportion of over-hydrogenated products is increased.

[0143] Comparative example 4: Application of rhenium-tungsten nano-alloy cluster catalyst without MgO

[0144] The difference between the comparative example 4 and the example 1 is only that:

[0145] The lignin depolymerization application step: the reaction is carried out according to the method of the example 1, but no MgO powder is introduced in the second step.

[0146] The comparative example 4 lacks the neutralization effect of MgO on the strong acid sites on the catalyst surface, the probability of over-hydrogenation of monophenol products on the active center is increased, resulting in a decrease in monophenol selectivity, and a significant increase in the proportion of over-hydrogenated products in the products.

[0147] The performance comparison experimental data of the above examples and comparative examples are as follows:

[0148]

[0149] The experimental data shows that: from the data of examples 1-3, the nano-alloy cluster catalyst with tungsten core-rhenium shell structure, combined with segmented temperature control (210-230℃ preferentially ruptures β-O-4 bond, 230-250℃ promotes the generation of monophenol) and MgO regulation, the lignin conversion rate is stable at 65%-72%, the monophenol selectivity is 80%-85%, the β-O-4 bond rupture efficiency (relative value) is 88%-100%, and the proportion of target monophenols such as guaiacol and eugenol is 59%-65%. The strong electron transfer ability of tungsten weakens the β-O-4 bond energy (reduces by 30%-40%) through d-orbital electron transfer, significantly improves the depolymerization efficiency, and solves the problem of low depolymerization efficiency caused by stable chemical bonds in traditional methods; the high hydrogenation activity of rhenium stabilizes the intermediate after rupture by providing hydrogen protons, combined with the directionality of the core-shell structure, effectively avoids the random reaction (such as over-hydrogenation, polymerization) of the intermediate, reduces the generation of by-products, and solves the problem of complex products. At the same time, MgO neutralizes the strong acid sites on the catalyst crystal face (100), reduces the probability of over-hydrogenation of monophenol on the active center (more than 40% reduction), and further ensures the directional retention of target products; the segmented temperature control mechanism matches the needs of different reaction stages, maximizes the selectivity of monophenol generation while efficiently rupturing the β-O-4 bond, and the two form a synergistic effect with the core-shell structure to improve the catalytic performance. In addition, the activity retention rate of the catalyst after 5 cycles is still 85%-87%, which reflects the high stability of the core-shell structure (lattice distortion rate ≤5%), providing reliable support for industrial application.

[0150] In contrast, in the comparative examples, the comparative example 1 uses a rhenium-tungsten mixed catalyst with no core-shell structure, and due to the dispersion of active sites, the lignin conversion rate is only 38%, the monophenol selectivity is 55%, and the β-O-4 bond breaking efficiency (relative value) is only 50%. These data show that the core-shell structure is a necessary condition for realizing the synergistic effect of tungsten-rhenium and improving the catalytic efficiency; the comparative example 2 uses a single tungsten component catalyst without introducing MgO, and due to the lack of rhenium hydrogenation activity to stabilize the intermediate, the monophenol selectivity is only 45%, and the by-products such as polycyclic aromatic hydrocarbons account for 60%, which confirms the irreplaceability of rhenium in reducing by-products and improving product directionality; the comparative example 3 uses a single temperature reaction, which cannot realize the precise matching of β-O-4 bond breaking and monophenol stability, and the monophenol selectivity is reduced to 60%, and the excessive hydrogenation products (such as cyclohexanol) account for 30%, which reflects the importance of segmented temperature control in inhibiting side reactions; the comparative example 4 does not introduce MgO, although the β-O-4 bond breaking efficiency is close to the example (98%), but the monophenol selectivity is only 60%, and the excessive hydrogenation products account for 35%, which directly proves the key role of MgO in neutralizing strong acid sites and inhibiting excessive hydrogenation.

[0151] It will be obvious to a person skilled in the art that the application is not limited to the details of the above-described exemplary embodiments, but that the application can be implemented in other concrete forms without departing from the spirit or essential characteristics of the application. Therefore, the embodiments should be considered in all respects as illustrative and not restrictive, and the scope of the application is defined by the appended claims rather than the above description, and it is intended to encompass all changes falling within the meaning and scope of the equivalent elements of the claims.

Claims

1. A rhenium-tungsten nanoalloy cluster catalyst, characterized in that, The catalyst consists of a mesoporous Al2O3 support and tungsten core-rhenium shell nanoalloy clusters supported thereon, wherein the atomic ratio of rhenium to tungsten is 1:3-1:

5. The catalyst is prepared by the following steps: Step 1: Take the mesoporous Al2O3 support, grind and sieve it, wash it with deionized water and dry it to obtain the pretreated mesoporous Al2O3 support for later use. Step 2: Add tungsten core-rhenium shell structured nanoalloy clusters to anhydrous ethanol and ultrasonically disperse them to form a uniform suspension; According to the loading ratio of rhenium and tungsten accounting for 5%-10% of the mass of the mesoporous Al2O3 support, the pretreated mesoporous Al2O3 support was added to the above suspension. The mixture was stirred to uniformly load the tungsten core-rhenium shell structured nanoalloy clusters in the suspension onto the support surface. Then, the ethanol in the system was evaporated to remove the ethanol, and a solid precursor loaded with tungsten core-rhenium shell structured nanoalloy clusters was obtained. The tungsten core-rhenium shell structured nanoalloy clusters have a tungsten atom content of ≥80% in the core region and a rhenium atom content of ≥70% in the shell region; they include three crystal lattice structures: body-centered cubic, face-centered cubic, and hexagonal close-packed, with active sites accounting for ≥60% at the (110) crystal plane interface; the rhenium atom distribution density at the (110) crystal plane interface is more than 50% higher than that at the center of the (100) crystal plane, and the diffusion depth is 0.5-2 atomic layers; after 1ns relaxation treatment, the lattice distortion rate is ≤5%; Step 3: Place the solid precursor obtained in Step 2 into a tube furnace, heat it to 280-320℃ in an air atmosphere and hold it at that temperature to remove residual impurities and obtain the calcined product. Step 4: Switch the air atmosphere in the tube furnace to a hydrogen atmosphere, heat to 420-480℃ and hold, cool and then introduce nitrogen for protection to obtain the activated product. Step 5: Grind and sieve the activated product obtained in Step 5 to obtain rhenium-tungsten nano-alloy cluster catalyst.

2. The rhenium-tungsten nanoalloy cluster catalyst according to claim 1, characterized in that, In step one, the mesoporous Al2O3 support is ground and passed through a 200-300 mesh sieve, washed with deionized water 3-5 times, dried at 100-120℃ for 10-14 hours. In step two, the ultrasonic dispersion time is 20-40 minutes, the power is 250-350W, the frequency is 35-45kHz, the stirring rate is 250-350r / min, the stirring time is 2-4 hours, and the temperature for evaporating and removing ethanol is 70-90℃.

3. The rhenium-tungsten nanoalloy cluster catalyst according to claim 1, characterized in that, In step three, the heating rate in air atmosphere is 4-6℃ / min, and the holding time is 1.5-2.5 hours; In step four, the flow rate of the hydrogen atmosphere is 40-60 mL / min, the heating rate is 1-3℃ / min, and the holding time is 2-4 hours.

4. The rhenium-tungsten nanoalloy cluster catalyst according to claim 1, characterized in that, In step five, the activated product is ground and passed through an 80-120 mesh sieve to obtain a rhenium-tungsten nano-alloy cluster catalyst; in the catalyst, rhenium is present in the form of Re. 4+ Mainly, accounting for 60%-70%, tungsten is mainly W 6+ It is the main component, accounting for 75%-85%.

5. The catalytic application of the rhenium-tungsten nanoalloy cluster catalyst according to any one of claims 1-4 in the depolymerization of lignin to prepare phenolic compounds, characterized in that, Includes the following steps: Step 1: Grind the lignin raw material into a particle size of 50-100μm by ball milling, and then sieve it for later use; Step 2: Mix the lignin treated in Step 1 with the rhenium-tungsten nano-alloy cluster catalyst at a mass ratio of 10:

1. Add methanol-water mixed solvent to the mixture, wherein the volume ratio of methanol to water is 3:

1. Introduce MgO powder accounting for 0.1%-0.5% of the catalyst mass. Stir until the lignin, catalyst and MgO are uniformly dispersed in the solvent. Adjust the pH of the system to 3-4 with dilute sulfuric acid. Step 3: Transfer the mixture obtained in Step 2 to a high-pressure reactor, seal it, and then introduce H2 to replace the air inside the reactor, maintaining the H2 pressure at 2-3 MPa. During the reaction, a segmented temperature control method is used. The reaction is first heated to 210-230℃ for 1.5-2.5 hours to preferentially break the β-O-4 bond in the lignin molecule, and then heated to 230-250℃ for another 1.5-2.5 hours to promote the formation of monophenolic compounds. The entire reaction is carried out at a stirring rate of 500-800 rpm. By controlling the temperature and time of the two-stage reaction, the excessive hydrogenation of the monophenolic products can be reduced. Step 4: After the reaction is completed, the mixture in the high-pressure reactor is cooled to room temperature and the remaining pressure in the reactor is released. The catalyst is recovered from the reaction mixture by centrifugation. The remaining reaction liquid is subjected to vacuum distillation to remove the methanol-water mixed solvent. The crude product is purified by silica gel column chromatography. The target fraction containing monophenolic compounds is collected and dried to obtain monophenolic compounds.

6. The catalytic application according to claim 5, characterized in that, In step one, the lignin raw material is herbaceous plant straw, which is ball-milled for 2-4 hours using zirconium oxide grinding balls at a ball-to-material ratio of 10:1, and then screened through a 150-300 mesh standard sieve. In step two, the stirring is performed using magnetic stirring at 300-500 rpm for 30-60 minutes; the concentration of dilute sulfuric acid is 0.1-0.5 mol / L.

7. The catalytic application according to claim 5, characterized in that, In step three, the H2 purity is 99.99%, and the heating rate is 5-10℃ / min; the generated monophenolic intermediate contains at least guaiacol, eugenol, and p-ethylphenol.

8. The catalytic application according to claim 5, characterized in that, In step three, the lignin conversion rate is 65%-75%, and the selectivity of monophenolic compounds is ≥80%; among the monophenolic compounds, guaiacol accounts for 30%-40% and syringol accounts for 25%-35%.

9. The catalytic application according to claim 5, characterized in that, In step four, the centrifugation speed is 8000-10000 rpm and the time is 10-15 min; the vacuum degree of vacuum distillation is 0.08-0.1 MPa and the temperature is 60-80℃, using a rotary evaporator with a speed of 50-100 rpm. The silica gel used in the silica gel column chromatography has a particle size of 100-200 mesh, a column diameter-to-height ratio of 1:8-1:12, and petroleum ether-ethyl acetate as the eluent with a volume ratio of 5:

1. The drying process is carried out under vacuum at 40-60℃ for 12-24 hours to obtain monophenolic compounds with a purity ≥95%.

10. The catalytic application according to claim 5, characterized in that, It also includes a catalyst regeneration step: the catalyst recovered by centrifugation is first ultrasonically washed with 5-10 times its weight of anhydrous ethanol at a power of 300W for 30 minutes, then soaked in 0.05mol / L H2SO4 solution for 1 hour to dissolve the residue, and finally washed with deionized water until neutral. After drying at 120℃ for 2-4 hours, it is reactivated at 400℃ in an H2 atmosphere for 2 hours to restore Re. 4+ and W 6+ The active valence state.

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