Synthesis method of monometal doped heteropolyacid catalyst and application of monometal doped heteropolyacid catalyst in lignin conversion

Abstract

The invention discloses a synthesis method of a monometal doped heteropolyacid catalyst and application of the monometal doped heteropolyacid catalyst in lignin conversion, and belongs to the technical field of catalysts.The preparation method of the monometal doped heteropolyacid catalyst specifically comprises the following steps that metal salt and cesium carbonate are mixed to obtain a heterogeneous solution, the heterogeneous solution is dropped into a heteropolyacid solution to be subjected to a mixed reaction, and the monometal doped heteropolyacid catalyst is obtained. The monometal doped heteropolyacid catalyst is obtained; the metal salt is Pd salt or Co salt. The method has profound significance in the aspect of high-value utilization of biomass resources, and provides powerful technical support for promoting sustainable development of biomass energy and chemical industry.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomass degradation, and particularly relates to a synthesis method of a single-metal-doped heteropolyacid catalyst and its application in lignin conversion. Background Art

[0002] Lignocellulosic biomass, as a rich renewable carbon source in nature and also a natural phenolic renewable resource, is an excellent choice for producing fine chemicals and fuels and is regarded as an ideal substitute for fossil energy. It consists of three major biopolymers: cellulose, hemicellulose, and lignin. Among them, the monosaccharides obtained after hydrolysis of cellulose and hemicellulose have been widely used in many fields. As an indispensable part of lignocellulose, lignin is even known as the only renewable aromatic carbon source, with rich reserves and abundant aromatic compounds. However, due to its stable and complex polymer structure, the enhancement of lignin value faces challenges in both the traditional pulp industry and the emerging second-generation biofuel industry.

[0003] Efficiently converting lignin into high-value chemicals can not only greatly improve the utilization efficiency of biomass resources but also meet the urgent need for sustainable development and promote the smooth transition from traditional fossil energy to biomass energy. Therefore, realizing the efficient conversion of lignin has become the core of developing lignin processing and recycling technologies and has far-reaching significance for its application in the energy and chemical industries. Among the linkage bonds of lignin, the β-O-4 bond dominates, while the Cβ-O bond and the Cα-Cβ bond are the main cleavage targets during the lignin depolymerization process. Oxidative cleavage has been proven to be a very effective method for cleaving the β-O-4 bond of lignin, which can generate aromatic compounds retaining active functional groups. In recent years, research on lignin depolymerization triggered by the formation of oxidative ketones in the β-O-4 unit has revealed that the formation of carbonyl groups can reduce the bond energy of the connected Cβ-O bond, thereby promoting the subsequent cleavage of the β-O-4 bond. Designing and applying environmentally friendly and high-performance catalysts provides a very promising solution for promoting the formation of oxidative ketones, cleaving the β-O-4 bond of lignin, and thus achieving the efficient conversion of lignin into high-value chemicals.

[0004] The patent document with publication number CN116196950A discloses a synthesis method of a double-metal-doped heteropolyacid catalyst and its application in lignin conversion, in which Ni and Co are doped into the heteropolyacid catalyst. However, the technical principle of this technical solution is to selectively cleave the C-O bond on the aryl ether and then avoid the recondensation of lignin caused by the coupling of C-C bonds after the cleavage of the C-O bond, rather than focusing on promoting the formation of oxidative ketones.

[0005] Therefore, how to provide a method for efficiently converting lignin into high-value chemicals is a technical problem that those skilled in the art urgently need to solve. Summary of the Invention

[0006] To solve the above technical problems, the present invention proposes a synthesis method of a single-metal-doped heteropolyacid catalyst and its application in lignin conversion.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A preparation method of a single-metal-doped heteropolyacid catalyst, which comprises mixing a metal salt and cesium carbonate to obtain a heterogeneous solution, and then dropping it into a heteropolyacid solution for mixing reaction to obtain the single-metal-doped heteropolyacid catalyst;

[0009] The metal salt is a Pd salt or a Co salt.

[0010] Beneficial effects: By introducing Pd or Co metal, the present invention adjusts the redox property and acidity of the catalyst, thereby affecting the reaction activity of the catalyst and increasing the yield of lignin degradation products. In addition, the addition of cesium carbonate endows the catalyst with good recycling characteristics, which conforms to the essence of green chemistry. This method has mild conditions and a simple operation method, and has a positive significance for increasing the yield of lignin degradation products.

[0011] Preferably, the Pd salt is Pd(NO3)2·2H2O;

[0012] The Co salt is Co(NO3)2·6H2O.

[0013] Beneficial effects: By introducing Pd or Co metal, the present invention adjusts the redox property and acidity of the catalyst, thereby affecting the reaction activity of the catalyst and increasing the yield of lignin degradation products.

[0014] Preferably, the heteropolyacid solution is obtained by dissolving phosphomolybdic acid (H3PMo 12 O 40 ) in acidified deionized water.

[0015] Beneficial effects: By introducing phosphomolybdic acid (H3PMo 12 O 40 ), the present invention promotes the cleavage of the β-O-4 bond of lignin and increases the yield of aromatic monomers.

[0016] Preferably, the dropping is carried out while stirring under ice bath conditions;

[0017] The stirring rate is 200 rpm.

[0018] More preferably, the mixing reaction is carried out under stirring conditions, the stirring rate is 200 rpm, and the temperature is low temperature 7±3°C.

[0019] Advantageous effects: The stirring conditions described in the present invention are beneficial to the dissolution of heteropolyacid and the full integration with the metal salt solution.

[0020] Preferably, the molar ratio of the heteropolyacid, cesium carbonate and the metal salt is 0.8:1.2:(0.08 - 0.12).

[0021] The present invention also provides a single-metal-doped heteropolyacid catalyst prepared by the above preparation method.

[0022] The present invention also provides the application of the above single-metal-doped heteropolyacid catalyst in the conversion of lignin.

[0023] Preferably, the application method of the single-metal-doped heteropolyacid catalyst in the conversion of lignin includes the following steps:

[0024] Mix lignin and the single-metal-doped heteropolyacid catalyst in a solvent for a reaction. After the reaction is completed, cool and centrifuge. Remove the solvent from the obtained supernatant and then extract. Collect the organic phase, and then dry, filter, and remove the solvent from the obtained organic phase to obtain a lignin oily product.

[0025] Advantageous effects: The present invention realizes the efficient directional conversion of lignin by optimizing the catalyst design and subsequent treatment processes, and has the advantages of catalyst recyclability, low environmental burden and economic feasibility, providing a new way for the sustainable conversion of biomass into valuable chemicals.

[0026] Preferably, the lignin is the lignin obtained after removing cellulose and hemicellulose from biomass.

[0027] Preferably, the mass ratio of the lignin to the single-metal-doped heteropolyacid catalyst is 1∶1;

[0028] Preferably, the temperature of the mixing reaction is 150°C and the time is 180 min.

[0029] More preferably, the mixing reaction is carried out under stirring conditions, and the stirring speed is 600 rpm.

[0030] Preferably, the solvent is a mixed solvent of methanol and water, and the volume ratio of methanol to water is 8:2.

[0031] Advantageous effects: The conditions in the high-pressure reaction kettle of the present invention are mild and the operation is simple. Using green solvents is beneficial to reducing the environmental burden.

[0032] Compared with the prior art, the present invention has the following advantages and technical effects:

[0033] The present invention introduces a single metal element to finely regulate the redox performance and acidic characteristics of the catalyst, thereby profoundly affecting the reaction activity of the catalyst and achieving a significant increase in the yield of lignin degradation products. Notably, the addition of cesium carbonate in the present invention not only optimizes the performance of the catalyst but also endows it with excellent recycling characteristics, and this innovative design perfectly conforms to the core concept of green chemistry.

[0034] The present invention opens up a new catalyst synthesis route for the efficient degradation and conversion of lignin into aromatic monomers. The entire lignin degradation process is carried out under mild conditions in a high-pressure reactor, and the operation process is simple and easy. It not only greatly improves the yield of lignin degradation products but also brings new breakthroughs to the catalytic conversion technology of lignin, which is conducive to the high-value utilization of biomass resources. Brief Description of the Drawings

[0035] The drawings forming a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application. In the drawings:

[0036] Figure 1 It is the GC-MS diagram of lignin degradation under the catalytic conditions of Pd / CsPMA obtained in Example 1;

[0037] Figure 2 It is the GC-MS diagram of lignin degradation under the catalytic conditions of CsPMA;

[0038] Figure 3 It is the mechanism diagram of lignin degradation under the catalytic conditions of Pd / CsPMA obtained in Example 1 or Co / CsPMA obtained in Example 2 of the present invention. Detailed Description of the Embodiments

[0039] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0040] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0041] Unless otherwise specified, room temperature or normal temperature in the embodiments of the present invention both refer to 25 ± 3 °C;

[0042] Low temperature in the embodiments of the present invention all refers to 7 ± 3 °C.

[0043] In the embodiments of the present invention, the lignin used is pine lignin, and the preparation method is as follows: The crushed pine wood powder is ball-milled for 4 h, and then degreasing treatment is carried out. The degreasing treatment method is: wrap the birch powder with degreasing filter paper, and then put it into a Soxhlet extractor. Add organic solvents benzene and ethanol (2:1) to the bottom of the round-bottom flask and place it in an oil bath. Set the temperature to 90 °C and extract in a fume hood for 6 h, ensuring that the extraction frequency per hour is between 4 and 6 times. After the extraction is completed, dry the pine wood powder for later use. The pine lignin is extracted by the organic solvent method. Take 50 g of the ball-milled and degreased pine powder and place it in a three-necked flask. Pour 298 mL of 1,4-dioxane and 8.5 mL of hydrochloric acid (37 wt%) into the three-necked flask and mix well. Then place the three-necked flask in an oil bath, stir and heat while condensing and refluxing for 4 h. After the reaction is completed, naturally cool it to room temperature. Then add 16.8 g of NaHCO3 and stir for 0.5 h. Use vacuum filtration to separate the solid and liquid and retain the mixed liquid. After the separation is completed, distill the filtrate under reduced pressure in a water bath at 40 °C until a concentrated solution is formed. Then add 150 mL of ethyl acetate to dilute the concentrated solution and drop it into 500 mL of n-hexane. A precipitate appears, filter it, and finally add 1000 mL of distilled water to wash and dry it. The obtained solid product is lignin.

[0044] Unless otherwise specified, other raw materials in the embodiments of the present invention are all obtained through commercial channels.

[0045] Example 1

[0046] A synthesis method of a single-atom doped heteropolyacid catalyst includes the following steps:

[0047] (1) Dissolve 1.5 g (0.814 mmol) of PMA (molybdophosphate) in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0048] (2) Dissolve 390 mg (1.2 mmol) of Cs2CO3 and 28 mg (0.0984 mmol) of Pd(NO3)2·2H2O in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cation stock solution.

[0049] (3) Drop the above cation solution into the heteropolyacid solution drop by drop under the conditions of ice bath and stirring at 200 rpm. During this process, a yellow precipitate gradually appears to form a mixed suspension. Continue to stir at low temperature for 5 h. After the stirring is completed, centrifuge to separate the solid, wash it three times with deionized water, dry it with a freeze dryer, and then grind it to obtain the product Pb / CsPMA.

[0050] Example 2

[0051] A method for synthesizing a single-atom doped heteropolyacid catalyst, comprising the following steps:

[0052] (1) Dissolve 1.5 g of PMA in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0053] (2) Dissolve 390 mg of Cs2CO3 and 30 mg of (Co(NO3)2·6H2O) in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cationic stock solution.

[0054] (3) Dropwise add the above cationic solution into the heteropolyacid solution under the conditions of an ice bath and stirring at 200 rpm. During this process, a yellow precipitate gradually appears to form a mixed suspension. Continue stirring at low temperature for 5 h. After the stirring is completed, centrifuge to separate the solid, wash it three times with deionized water, dry it with a freeze dryer, and then grind it to obtain the product Co / CsPMA.

[0055] Application Example 1

[0056] An application of a single-atom doped heteropolyacid catalyst in lignin conversion, comprising the following steps:

[0057] (1) Mix 0.25 g of pine lignin, 0.25 g of Pd / CsPMA obtained in Example 1, and 10 mL of a methanol / water solvent (volume ratio 8:2), and react in a high-pressure reactor at 150 °C, 600 rmp, and with 1 MPa of O2 introduced for 180 min. After the reaction is completed, cool for 1 h, take out the reaction solution, filter to remove the residue in a funnel device, wash the residual solution in the reactor with a methanol solution, and perform rotary evaporation using a rotary evaporator to evaporate the methanol. Set the temperature to 55 °C. Then, repeatedly extract the evaporated product three times with 20 mL of dichloromethane and 20 mL of distilled water to obtain an organic phase. Add anhydrous magnesium sulfate to remove the excess water, and perform rotary evaporation again using a rotary evaporator to evaporate the dichloromethane. Set the temperature to 55 °C to obtain a lignin oily product.

[0058] (2) Add the obtained lignin oily product to 10 mL of ethyl acetate, store it in a centrifuge tube, filter the reaction solution again using a filter needle, and then add an internal standard using a pneumatic air gun. The internal standard is prepared as follows: Dissolve 0.0144 g of 3,5-dimethylphenol in 10 mL of chromatographically pure ethyl acetate.

[0059] (3) Perform gas chromatography - mass spectrometry analysis on the solution prepared in the previous step. The instrument model used is Agilent 6890N / 5973i. The required detection conditions are as follows: the chromatographic column is HP - 5MS capillary column, and the programmed temperature rise is as follows: initially at 70 °C for 2 min, then rising to 180 °C at a rate of 7 °C / min and maintaining for 8 min. The detector is FID (200 °C), the internal standard is 3,5 - dimethoxyphenol; the carrier gas is helium, the split ratio is 5.01∶1, the injection port temperature is 180 °C, and the injection volume is 1 μL. The total yield of monomer products during lignin degradation is 11.01%. Figure 1 It is the GC - MS diagram of lignin degradation under the catalytic condition of Pd / CsPMA, from which the composition distribution of aromatic monomers can be seen.

[0060] Application Example 2

[0061] The application of a single - atom - doped heteropolyacid catalyst in lignin conversion includes the following steps:

[0062] Mix 0.25 g of pine lignin, 0.25 g of Co / CsPMA obtained in Example 2, and 10 mL of methanol / water solvent (volume ratio 8∶2). React in a high - pressure reactor at 150 °C, 600 rmp, and with 1 MPa of O2 introduced for 180 min. After the reaction, cool for 1 h. Take out the reaction solution and filter to remove residues in a funnel device, and wash the residual solution in the reactor with methanol solution. Use a rotary evaporator to perform rotary evaporation to evaporate methanol, set the temperature to 55 °C. Then, extract the evaporated product three times repeatedly with 20 mL of dichloromethane and 20 mL of distilled water to obtain the organic phase. Add anhydrous magnesium sulfate to remove excess water, and use a rotary evaporator again to perform rotary evaporation to evaporate dichloromethane, set the temperature to 55 °C, to obtain a lignin oily product. Other process steps and parameters are the same as those in Application Example 1. The total yield of monomer products during lignin degradation is 10.33%.

[0063] Comparative Example 1

[0064] A synthesis method of a heteropolyacid catalyst includes the following steps:

[0065] (1) Dissolve 1.5 g of PMA in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0066] (2) Dissolve 390 mg of Cs2CO3 in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cation stock solution.

[0067] (3) The above cation solution was added dropwise into the heteropolyacid solution under the conditions of an ice bath and stirring at 200 rpm to form a mixed suspension, and stirring was continued at low temperature for 5 h. After the stirring was completed, the solid was separated by centrifugation, washed three times with deionized water, dried with a freeze dryer, and then ground to obtain the product CsPMA.

[0068] An application of a heteropolyacid catalyst in lignin conversion includes the following steps:

[0069] (1) 0.25 g of pine lignin, 0.25 g of CsPMA, and 10 mL of a methanol / water solvent (the volume ratio of methanol to water is 8:2) were mixed and reacted in a high-pressure reactor at 150 °C, 600 rmp, and with 1 MPa of O2 introduced for 180 min. After the reaction was completed, it was cooled for 1 h. The reaction solution was taken out and filtered in a funnel device to remove the residue, and the residual solution in the reactor was washed with a methanol solution. Then, rotary evaporation was carried out at 55 °C using a rotary evaporator to remove methanol. The evaporated product was repeatedly extracted three times with 20 mL of dichloromethane and 20 mL of distilled water to obtain an organic phase. Anhydrous magnesium sulfate was added to remove the excess water, and rotary evaporation was carried out again using a rotary evaporator to remove dichloromethane, with the temperature set at 55 °C, to obtain a lignin oily product. All other process steps and parameters were the same as those in Application Example 1. The total yield of monomer products during lignin degradation was 6.42%.

[0070] Figure 2 It is the GC-MS diagram of lignin degradation under the catalytic conditions of CsPMA. It can be seen that the composition distribution of aromatic monomers and their retention times are the same as those of the monomer products in Application Example 1.

[0071] Comparative Example 2

[0072] A synthesis method of a heteropolyacid catalyst includes the following steps:

[0073] (1) 4.6 g of PTA (H3PW 12 O 40 ) was dissolved in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0074] (2) 390 mg of Cs2CO3 was dissolved in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cation stock solution.

[0075] (3) The above cation solution was added dropwise into the heteropolyacid solution under the conditions of an ice bath and stirring at 200 rpm to form a mixed suspension, and stirring was continued at low temperature for 5 h. After the stirring was completed, the solid was separated by centrifugation, washed three times with deionized water, dried with a freeze dryer, and then ground to obtain the product CsPTA.

[0076] Application of a heteropolyacid catalyst in lignin conversion, comprising the following steps:

[0077] Mix 0.25 g of pine lignin, 0.25 g of CsPTA, and 10 mL of methanol / water solvent (volume ratio 8:2), react in a high-pressure reactor at 150 °C, 600 rmp, and with 1 MPa of O2 introduced for 180 min. After the reaction, cool for 1 h, take out the reaction solution and filter to remove residues in a funnel device, wash the residual solution in the reactor with methanol solution, perform rotary evaporation using a rotary evaporator to evaporate methanol, set the temperature to 55 °C, then repeatedly extract the evaporated product three times with 20 mL of dichloromethane and 20 mL of distilled water to obtain an organic phase, add anhydrous magnesium sulfate to remove excess water, and perform rotary evaporation again using a rotary evaporator to evaporate dichloromethane, set the temperature to 55 °C, to obtain a lignin oily product. Other process steps and parameters are the same as those in Application Example 1. The total yield of monomer products during lignin degradation is 5.72%.

[0078] Comparative Example 3

[0079] Synthesis method of a heteropolyacid catalyst, comprising the following steps:

[0080] (1) Dissolve 4.6 g of PTA (phosphotungstic acid) in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0081] (2) Dissolve 390 mg of Cs2CO3 and 28 mg of Pd(NO3)2·2H2O in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cation stock solution.

[0082] (3) Dropwise add the above cation solution into the heteropolyacid solution under ice bath and stirring conditions of 200 rpm to form a mixed suspension, continue stirring at low temperature for 5 h, after the stirring ends, centrifuge to separate the solid, then wash three times with deionized water, dry with a freeze dryer, and grind to obtain the product Pb / CsPTA.

[0083] Application of a heteropolyacid catalyst in lignin conversion, comprising the following steps:

[0084] Mix 0.25 g of pine lignin, 0.25 g of Pd / CsPTA, and 10 mL of methanol / water solvent (volume ratio 8:2). React in a high-pressure reactor at 150 °C, 600 rmp, and with 1 MPa of O2 introduced for 180 min. After the reaction, cool for 1 h. Take out the reaction solution and filter to remove residues in a funnel device. Wash the residual solution in the reactor with methanol solution. Use a rotary evaporator to perform rotary evaporation to evaporate methanol, set the temperature to 55 °C. Then, repeatedly extract the evaporated product three times with 20 mL of dichloromethane and 20 mL of distilled water to obtain the organic phase. Add anhydrous magnesium sulfate to remove excess water. Use the rotary evaporator again for rotary evaporation to evaporate dichloromethane, set the temperature to 55 °C, to obtain a lignin oily product. All other process steps and parameters are the same as in Application Example 1. The total yield of monomer products during lignin degradation is 6.59%.

[0085] Comparative Example 4

[0086] A method for synthesizing a heteropolyacid catalyst includes the following steps:

[0087] (1) Dissolve 1.5 g of PMA in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to obtain a heteropolyacid solution.

[0088] (2) Dissolve 390 mg of Cs2CO3 and 30 mg of (Ni(NO3)2·6H2O) in 30 mL of acidified deionized water (obtained by dropping two drops of concentrated HNO3 into deionized water) to prepare a cation stock solution.

[0089] (3) Drop the above cation solution into the heteropolyacid solution drop by drop under ice bath and stirring conditions of 200 rpm. During this process, a yellow precipitate gradually appears to form a mixed suspension. Continue stirring at low temperature for 5 h. After stirring, centrifuge to separate the solid, then wash it three times with deionized water, dry it with a freeze dryer, and grind it to obtain the product Ni / CsPMA.

[0090] An application of a heteropolyacid catalyst in lignin conversion includes the following steps:

[0091] 0.25g of pine lignin, 0.25g of Ni / CsPMA, and 10mL of methanol / water solvent (volume ratio of 8:2) were mixed and reacted in a high-pressure reactor at 150°C, 600rmp, and 1MPa O2 for 180min. After the reaction was completed, the reaction solution was taken out and filtered in a funnel device to remove the residue, and the residual solution in the reactor was washed with a methanol solution. A rotary evaporator was used to perform rotary evaporation to evaporate the methanol, and the temperature was set to 55°C. The evaporated product was repeatedly extracted three times with 20mL of dichloromethane and 20mL of distilled water to obtain an organic phase, and anhydrous magnesium sulfate was added to remove excess water. The rotary evaporator was used again to perform rotary evaporation to evaporate the dichloromethane, and the temperature was set to 55°C to obtain a lignin oily product. The other process steps and parameters were the same as those in Application Example 1. The total yield of monomer products when degrading lignin was 6.56%.

[0092] Comparative Example 5

[0093] A method for synthesizing a heteropolyacid catalyst comprises the following steps:

[0094] (1) Dissolve 1.5 g of PMA in 30 mL of acidified deionized water (obtained by adding two drops of concentrated HNO3 to deionized water) to obtain a heteropolyacid solution.

[0095] (2) 390 mg of Cs2CO3 and 42 mg of Fe(NO3)3·9H2O were dissolved in 30 mL of acidified deionized water (obtained by adding two drops of concentrated HNO3 to deionized water) to prepare a cationic stock solution.

[0096] (3) The cationic solution was added dropwise into the heteropolyacid solution under ice bath and stirring conditions at 200 rpm. During this process, a yellow precipitate gradually appeared to form a mixed suspension. The mixture was stirred at low temperature for 5 h. After stirring, the solid was separated by centrifugation, washed three times with deionized water, dried in a freeze dryer, and then ground to obtain the product Fe / CsPMA.

[0097] An application of a heteropolyacid catalyst in lignin conversion comprises the following steps:

[0098] 0.25g of pine lignin, 0.25g of Fe / CsPMA, and 10mL of methanol / water solvent (volume ratio of 8:2) were mixed, reacted in a high-pressure reactor at 150°C, 600rmp, and 1MPa O2 for 180min, cooled for 1h after the reaction, the reaction solution was taken out and filtered in a funnel device to remove the residue, and the residual solution in the reactor was washed with a methanol solution, and a rotary evaporator was used to perform rotary evaporation to evaporate the methanol, and the temperature was set to 55°C, and then the evaporated product was repeatedly extracted three times with 20mL of dichloromethane and 20mL of distilled water to obtain an organic phase, and then anhydrous magnesium sulfate was added to remove excess water, and a rotary evaporator was used again to perform rotary evaporation to evaporate the dichloromethane, and the temperature was set to 55°C to obtain a lignin oily product. Other process steps and parameters are the same as those in Application Example 1. The total yield of monomer products when degrading lignin is 8.7%.

[0099] Technical effects:

[0100] Figure 3 This is a diagram showing the mechanism of lignin degradation under catalytic conditions of Pd / CsPMA obtained in Example 1 and Co / CsPMA obtained in Example 2 (abbreviated as Pd(Co) / CsPMA). It can be seen that the present invention focuses on promoting the formation of oxidized ketones and cutting the β-O-4 bonds (including CO and CC bonds) of lignin.

[0101] The monomer products and yields of lignin degradation in Application Examples 1-2 and Comparative Examples 1-5 are shown in Table 1:

[0102] Table 1 Monomer products and yields (%) when degrading lignin

[0103]

[0104] As can be seen from Table 1, the degradation of lignin by the single metal doped heteropoly acid catalyst of the present invention has an advantage in the yield of monomer products over the degradation of lignin by the polyoxometalate without palladium, and the best effect is achieved when the reaction time is 180 min, the temperature is 150° C., and the catalyst is Pd / CsPMA. Therefore, the method of the present invention can promote the degradation of lignin and obtain a higher degradation yield.

[0105] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A preparation method of a single-metal-doped heteropolyacid catalyst, characterized in that A heterogeneous solution is obtained by mixing a metal salt with cesium carbonate, and then it is dropped into a heteropolyacid solution for mixing reaction to obtain the single-metal-doped heteropolyacid catalyst; The metal salt is a Pd salt or a Co salt.

2. The preparation method of a single-metal-doped heteropolyacid catalyst according to claim 1, characterized in that, The Pd salt is Pd(NO3)2·2H2O; The Co salt is Co(NO3)2·6H2O.

3. The preparation method of a single-metal-doped heteropolyacid catalyst according to claim 1, characterized in that The heteropolyacid solution is obtained by dissolving molybdenum phosphate in acidified deionized water.

4. The preparation method of a single-metal-doped heteropolyacid catalyst according to claim 1, characterized in that, The dropping is carried out while stirring under an ice bath condition; The stirring rate is 200 rpm.

5. The preparation method of a single metal-doped heteropolyacid catalyst according to claim 1, characterized in that, The molar ratio of the heteropolyacid, cesium carbonate and the metal salt is 0.8:1.2:(0.08 - 0.12).

6. A single-metal-doped heteropolyacid catalyst prepared by the preparation method according to claim 1.

7. An application of the single-metal-doped heteropolyacid catalyst according to claim 6 in lignin conversion.

8. The application according to claim 7, wherein It includes the following steps: The lignin and the single-metal-doped heteropolyacid catalyst are mixed and reacted in a solvent. After the reaction is completed, it is cooled and centrifuged. The obtained supernatant is extracted after removing the solvent, and the organic phase is collected. Then the obtained organic phase is dried, filtered and the solvent is removed to obtain the lignin oily product.

9. The application according to claim 8, wherein The mass ratio of the lignin to the single-metal-doped heteropolyacid catalyst is 1∶1.

10. The application according to claim 8, characterized in that, The temperature of the mixing reaction is 150°C and the time is 180 min.

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