Preparation method and application of acid-base carbon-based catalyst

Abstract

The present invention belongs to the technical field of catalyst preparation, and specifically relates to a preparation method and application of an acid-base carbon-based catalyst. A metal salt and water are prepared into a molten salt hydrate, and biomass is added to the molten salt hydrate for pretreatment; the pretreated biomass is roasted, washed, and dried to obtain an acid-base carbon-based catalyst. The present invention synthesizes an acid-base multifunctional carbon-based catalyst by pretreating biomass with molten salt hydrate, roasting, washing, and drying. The carrier raw material has a wide range of sources, and waste wheat straw, corn straw, pine sawdust and other biomass can be selected as raw materials; while preparing the carrier, the active component that efficiently catalyzes biomass carbohydrates to produce lactate is successfully loaded on the carrier.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation, and specifically relates to a preparation method and application of an acid-base carbon-based catalyst. Background Art

[0002] Lactate is a widely used chemical. It can not only be used as a green and environmentally friendly solvent, but also in the pharmaceutical, cosmetic and food industries. More importantly, lactate polymerization can synthesize polylactic acid, a biodegradable plastic with excellent biocompatibility. In 2020, the market demand for polylactic acid in my country is about 200,000 tons, and it is expected to grow at a rate of 15% to 25% in the future. The key to meeting the market demand for polylactic acid is to study the method of efficiently preparing lactate. At present, the production process of lactate is mainly to produce lactic acid by microbial fermentation of starch, and then the lactic acid is esterified with alcohol. However, in the process of microbial fermentation, there are problems such as low time and space yield, long production cycle, equipment corrosion, and only semi-continuous and intermittent production. In alcohol solvents, the preparation of lactate by catalytic conversion of biomass carbohydrates can effectively avoid the disadvantages of microbial fermentation, and has the advantages of low production cost and high efficiency. Therefore, it is of great significance to study the catalyst for the efficient catalytic conversion of biomass carbohydrates to lactate.

[0003] Chinese patent CN103058869 A discloses a method for preparing lactic acid and lactic acid esters by catalyzing sugar conversion. The patent uses Sn salt as a catalyst, an inorganic base as a pH regulator and NaCl as an auxiliary agent to catalyze glucose, sucrose, fructose, xylose and other biomass carbohydrates to prepare lactic acid and lactic acid esters. The catalyst used in this method has problems such as difficulty in recycling, corrosion of equipment and high product separation costs.

[0004] Molten salt hydrate is a widely used brine mixture. Molten salt hydrate can not only catalyze the conversion of biomass derivatives to produce levulinic acid (CN110950750 A), 5-hydroxymethylfurfural (CN113512016 A, CN113402485 A) and methyl thioglycolate (CN112521319 A), but also be used to catalyze the conversion of cellulose to produce lactic acid. Chinese patent CN113214072A discloses a method for converting cellulose to produce lactic acid in molten salt hydrate, which uses molten salt hydrates of AlCl3, CrCl3, FeCl3, CuCl2, ZnBr2 and MoCl3 as catalysts and reaction systems to catalyze the conversion of cellulose to produce lactic acid at 180-240°C. Chinese patent CN113979852 A discloses a method for catalyzing cellulose to produce lactic acid at low temperature and normal pressure using zinc chloride molten salt hydrate, which uses zinc chloride molten salt to treat cellulose to obtain regenerated cellulose, and uses Er-C as a catalyst to catalyze the regenerated cellulose to produce lactic acid. Chinese patent CN112745209 A discloses a method for preparing lactic acid from cellulose by combining zinc chloride molten salt hydrate with aluminum salt at low temperature and normal pressure. The method uses zinc chloride molten salt hydrate to dissolve cellulose and uses aluminum salt as a catalyst to catalyze the conversion of cellulose at 120-160°C. Molten salt hydrates are difficult to regulate the acid-base strength, and usually cause serious side reactions due to strong acidity; molten salt hydrates also have problems such as difficulty in recycling, equipment corrosion and product separation difficulties. More seriously, lactic acid is unstable and prone to further polymerization side reactions. Chinese patent CN111960414 A discloses a biomass-based porous carbon material prepared using inorganic molten salt hydrates, a preparation method and application. The patent uses concentrated inorganic acid and sodium chloride and potassium chloride to configure a strongly acidic molten salt hydrate, and then ultrasonically treats the biomass, freezes, freeze-dries, and carbonizes the porous carbon material; the method has cumbersome steps, and the inorganic acid seriously corrodes the equipment. Existing research has not yet involved the use of molten salt hydrates to prepare acid-base multifunctional carbon-based catalysts that efficiently catalyze biomass carbohydrates to lactate.

[0005] Chinese patent CN112062673A discloses a method for directional synthesis of methyl lactate by catalytic conversion of fructose in one pot, which uses Ga-Sn-HY molecular sieve as catalyst, catalyzes fructose reaction for 1 to 12 hours, and the yield of methyl lactate can reach 5.2% to 58.2%. Chinese patent CN113831238A discloses a method for catalytic conversion of carbohydrates to prepare methyl lactate, which uses A-SCM-1 molecular sieve as catalyst, A in A-SCM-1 molecular sieve is selected from at least one of Sn and Zr, and catalyzes glucose, fructose, xylose, sucrose, inulin, cellulose and starch reaction for 1 to 24 hours, respectively, and the selectivity of methyl lactate is 26.4% to 62.7%. However, the preparation of molecular sieve and the post-loading process of active components are complicated, and the preparation cycle is long, usually requiring 6 to 24 days, which limits its large-scale production and application; in addition, molecular sieve catalysts have problems such as low catalytic efficiency, easy coking and deactivation, and high preparation cost.

[0006] The reaction pathway for catalyzing the production of lactate from biomass carbohydrates is complex. Taking glucose as an example, glucose is first converted to fructose by isomerization, and then fructose is converted to trisaccharides (1,3-dihydroxyacetone and glyceraldehyde) by retro-aldol condensation. The trisaccharides are then dehydrated, acetalized, and rearranged to lactate. Each reaction step has different requirements for the number and strength of the acid-base active centers of the catalyst. Due to the single acid-base active center of the existing catalyst, the yield of catalyzing the production of lactate from biomass carbohydrates is low, the side reactions are serious, and the reaction efficiency is low. In order to efficiently catalyze the production of lactate from biomass carbohydrates, the catalyst is required to have suitable acid-base active centers at the same time; in order to achieve industrial application, the catalyst cost and preparation cycle need to be reduced. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing an acid-base carbon-based catalyst, which avoids the tedious steps of preparing the catalyst by the post-loading method, and successfully loads the active components that efficiently catalyze the production of lactic acid ester from biomass carbohydrates while preparing the carbon carrier. In addition, the prepared catalyst has the advantages of being recyclable, cheap and easy to obtain, having a short synthesis cycle and good hydrothermal stability. The present invention also provides the application of the acid-base carbon-based catalyst, which has the advantages of high catalytic efficiency, few side reactions and high lactic acid ester yield when used for catalyzing the production of lactic acid ester from biomass carbohydrates, and has good prospects for industrial application.

[0008] The preparation method of the acid-base carbon-based catalyst of the present invention comprises preparing a molten salt hydrate with metal salt and water, adding biomass to the molten salt hydrate for pretreatment; roasting, washing and drying the pretreated biomass to obtain the acid-base carbon-based catalyst.

[0009] The metal salt is one or more of zinc chloride, zinc bromide, zinc trifluoromethanesulfonate, magnesium chloride, lead chloride, ferric chloride, nickel chloride, stannous chloride, tin chloride or cobalt chloride, preferably one or more of zinc chloride, zinc bromide, zinc trifluoromethanesulfonate, ferric chloride or stannous chloride.

[0010] The molar ratio of the metal salt to water is 1:1-10.

[0011] The biomass is one or more of wheat straw, corn straw or pine sawdust.

[0012] The mass of the biomass is 1-20wt.%, preferably 5-10wt.%, of the mass of the molten salt hydrate.

[0013] The pretreatment is swelling.

[0014] The pretreatment temperature is 30-120° C., and the pretreatment time is 1-12 hours.

[0015] The calcination temperature is 300-500°C, and the calcination time is 3-6h.

[0016] The application of the catalyst prepared by the method for preparing the acid-base carbon-based catalyst of the present invention is to add the acid-base carbon-based catalyst and biomass carbohydrate into an alcohol solvent for reaction to obtain lactic acid ester.

[0017] The biomass carbohydrates are one or more of glucose, mannose or fructose.

[0018] The alcohol solvent is one of methanol, ethanol, propanol or n-butanol.

[0019] The mass ratio of the acid-base carbon-based catalyst to the biomass carbohydrate is 1:4-6.

[0020] The mass ratio of the biomass carbohydrate to the alcohol solvent is 1:140-160.

[0021] The reaction temperature is 160-240° C., and the reaction time is 15-120 min.

[0022] In the application of acid-base carbon-based catalysts, the acid-base carbon-based catalysts can be co-loaded with multiple metal salts, or can be mechanically mixed with acid-base carbon-based catalysts prepared with a single metal salt.

[0023] When the alcohol solvents are methanol, ethanol, propanol and n-butanol, the corresponding target products are methyl lactate, ethyl lactate, propyl lactate and butyl lactate.

[0024] The present invention adopts a method of dissolving a metal salt in water to obtain a molten salt hydrate, performs swelling pretreatment on biomass, and then obtains an acid-base carbon-based catalyst through high-temperature roasting, washing and drying.

[0025] The present invention pre-treats the biomass by swelling it with molten salt hydrate, so that the molten salt hydrate enters the interior of the cellulose; during the high-temperature roasting process, the cellulose is carbonized, and the molten salt hydrate inside the cellulose is hydrolyzed to generate a basic salt immobilized on the carbon. The prepared acid-base carbon-based catalyst has both acidic and basic active sites, the basic active sites are mainly provided by the hydroxyl groups coordinated by the basic salt metal, and the acidic active sites are provided by the Lewis acid at the center of the basic salt metal. When catalyzing carbohydrate reactions, taking glucose as an example, the basic active sites are mainly responsible for catalyzing the isomerization reaction of glucose and the retro-aldol condensation reaction of fructose, while the acidic active sites are mainly responsible for catalyzing the dehydration, acetalization and rearrangement reactions of tri-carbon sugars (1,3-dihydroxyacetone and glyceraldehyde).

[0026] Compared with molten salt hydrate, the catalyst of the present invention is insoluble in alcohol solvent and can be recycled and regenerated; it has both acid and alkaline active sites, which can reduce the generation of byproducts such as humin and improve the yield of lactate; in alcohol solvent, it catalyzes biomass carbohydrates to produce lactate in one step, avoiding the defects of lactic acid polymerization in water system; the product is lactate, and the product separation cost is low. Compared with molecular sieve catalysts, the present invention avoids the complicated operation steps of molecular sieve and metal active component post-loading, and can obtain a catalyst with both acidic and alkaline active sites through simple biomass swelling pretreatment and roasting and other operation steps; more importantly, the catalyst has higher catalytic efficiency and lactate yield.

[0027] The beneficial effects of the present invention are as follows:

[0028] 1. The present invention synthesizes an acid-base multifunctional carbon-based catalyst by pretreating biomass with molten salt hydrate, roasting, washing and drying. The carrier raw material has a wide range of sources, and can be selected from waste wheat straw, corn straw or pine sawdust and other biomass as raw materials; while preparing the carrier, the active component that efficiently catalyzes the production of lactate from biomass carbohydrates is successfully loaded on the carrier. The catalyst has the advantages of being recyclable, simple to prepare, short synthesis cycle, and low cost.

[0029] 2. By regulating the type of metal salt, the ratio of metal salt to biomass and the roasting temperature, a catalyst with both acid and base active centers can be prepared. When used to catalyze the production of lactate from biomass carbohydrates, it has the advantages of fast reaction rate, few side reactions and high lactate yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the SEM characterization image of the acid-base carbon-based catalyst Zn / C3 prepared in Example 3.

[0031] Figure 2 This is the TEM characterization image of the acid-base carbon-based catalyst Zn / C3 prepared in Example 3.

[0032] Figure 3 This is a characterization diagram of the N2 adsorption-desorption isotherm of the acid-base carbon-based catalyst Zn / C3 prepared in Example 3.

[0033] Figure 4 This is the CO2-TPD characterization diagram of the acid-base carbon-based catalyst Zn / C3 prepared in Example 3.

[0034] Figure 5 This is the NH3-TPD characterization diagram of the acid-base carbon-based catalyst Zn / C3 prepared in Example 3. DETAILED DESCRIPTION

[0035] The present invention is further described below with reference to the embodiments.

[0036] Example 1

[0037] Zinc chloride and water were prepared into molten salt hydrate in a molar ratio of 1:1, and 5wt.% wheat straw based on the molten salt hydrate was added, and the molten salt hydrate was swollen and pretreated at 30°C for 12h. The pretreated wheat straw was placed in a tubular furnace, heated to 300°C at a heating rate of 10°C / min for 3h under a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Zn / C1.

[0038] 0.04 g Zn / C1 and 0.16 g glucose were added to a high-pressure reactor, and 24 g methanol was added. The reaction was carried out at 160° C. for 120 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0039] Example 2

[0040] Zinc chloride and water were prepared into molten salt hydrate at a molar ratio of 1:10, and 10wt.% pine sawdust based on the molten salt hydrate was added, and the pine sawdust was pretreated for swelling at 120°C for 1h. The pretreated pine sawdust was placed in a tubular furnace, heated to 500°C at a heating rate of 10°C / min in a nitrogen atmosphere, and calcined for 6h, and finally washed with water and dried. The obtained catalyst was recorded as Zn / C2.

[0041] 0.04 g Zn / C2 and 0.24 g fructose were added to a high-pressure reactor, and 36 g ethanol was added. The reaction was carried out at 240° C. for 15 min in a 2 MPa nitrogen atmosphere. The yield of ethyl lactate is shown in Table 1.

[0042] Example 3

[0043] Zinc chloride and water were prepared into molten salt hydrate in a molar ratio of 1:3, and corn stalks with a content of 5 wt.% based on the molten salt hydrate were added and pre-treated at 80°C for 3 hours for swelling. The pre-treated corn stalks were placed in a tubular furnace, heated to 400°C for 3 hours at a heating rate of 10°C / min under a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Zn / C3.

[0044] The Zn / C3 was tested and the test results are shown in Figure 1-5 . SEM characterization ( Figure 1 ) showed that Zn / C3 has a rich pore structure, and TEM characterization ( Figure 2 ) clearly shows that the catalyst has a large number of mesopores and micropores. Figure 3 This is a characterization diagram of the N2 adsorption-desorption isotherm of the catalyst; the adsorption amount rises sharply in the low-pressure zone, indicating the existence of a large number of micropores, while the H3-type hysteresis loop appears in the medium-pressure section, indicating the existence of a large number of mesopores, which is consistent with the TEM characterization results. Figure 4 It is the CO2-TPD characterization of the catalyst. 100-200℃ and 200-400℃ correspond to the adsorption of CO2 by weak alkaline centers and strong alkaline centers, respectively, indicating that the catalyst has a large number of alkaline sites. NH3-TPD characterization ( Figure 5 ) clearly shows that the catalyst has both weak acid and strong acid centers. In summary, the prepared catalyst has both acid and base active centers and a large number of mesoporous and microporous structures.

[0045] 0.04 g Zn / C3 and 0.2 g mannose were added to a high-pressure reactor, and 30 g propanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of propyl lactate is shown in Table 1.

[0046] Example 4

[0047] Zinc bromide and water were prepared into molten salt hydrate in a molar ratio of 1:3, and corn stalks were added at 5wt.% of the molten salt hydrate, and swelled and pretreated at 80°C for 3h. The pretreated corn stalks were placed in a tubular furnace, heated to 400°C at a heating rate of 10°C / min for 3h in a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Zn / C4.

[0048] 0.04 g Zn / C4 and 0.2 g glucose were added to a high-pressure reactor, and 30 g n-butanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of butyl lactate is shown in Table 1.

[0049] Example 5

[0050] Zinc trifluoromethanesulfonate and water were prepared into molten salt hydrate in a molar ratio of 1:10, and corn stalks with a content of 5 wt.% based on the molten salt hydrate were added and pre-treated at 80°C for 3 hours for swelling. The pre-treated corn stalks were placed in a tubular furnace, heated to 400°C for 3 hours at a heating rate of 10°C / min under a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Zn / C5.

[0051] Zn / C3 and Zn / C5 were mechanically blended in a mass ratio of 3:1. 0.04 g of the mechanically blended catalyst and 0.2 g of fructose were added to a high-pressure reactor, and 30 g of methanol was added. The reaction was carried out at 200°C for 75 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0052] Example 6

[0053] Ferric chloride and water were prepared into molten salt hydrate in a molar ratio of 1:3, and corn stalks with a 5wt.% content based on the molten salt hydrate were added and pre-treated at 80°C for 3h for swelling. The pre-treated corn stalks were placed in a tubular furnace, heated to 400°C for 3h at a heating rate of 10°C / min under a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Fe / C.

[0054] 0.04 g Fe / C and 0.2 g fructose were added to a high-pressure reactor, and 30 g methanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0055] Example 7

[0056] Same as Example 6, except that ferric chloride was replaced by stannous chloride, and the obtained catalyst was recorded as Sn / C. The yield of methyl lactate is shown in Table 1.

[0057] Example 8

[0058] Same as Example 6, except that ferric chloride was replaced by magnesium chloride, and the obtained catalyst was recorded as Mg / C. The yield of methyl lactate is shown in Table 1.

[0059] Example 9

[0060] Same as Example 6, except that ferric chloride was replaced by lead chloride, and the obtained catalyst was recorded as Pb / C. The yield of methyl lactate is shown in Table 1.

[0061] Example 10

[0062] Same as Example 6, except that ferric chloride was replaced by nickel chloride, and the obtained catalyst was recorded as Ni / C. The yield of methyl lactate is shown in Table 1.

[0063] Embodiment 11

[0064] Same as Example 6, except that ferric chloride was replaced by cobalt chloride, and the obtained catalyst was recorded as Co / C. The yield of methyl lactate is shown in Table 1.

[0065] Example 12

[0066] Zinc chloride and ferric chloride were mixed in a molar ratio of 1:1, and the mixed salt and water were prepared in a molar ratio of 1:3 to prepare a molten salt hydrate, and 5wt.% corn stalks based on the molten salt hydrate were added, and the swell pretreatment was performed at 80°C for 3h. The pretreated corn stalks were placed in a tubular furnace, heated to 400°C at a heating rate of 10°C / min for 3h in a nitrogen atmosphere, and finally washed with water and dried. The obtained catalyst was recorded as Zn-Fe / C.

[0067] 0.04 g Zn-Fe / C and 0.2 g glucose were added to a high-pressure reactor, and 30 g methanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0068] Example 13

[0069] Same as Example 12, except that ferric chloride was replaced by stannous chloride, and the obtained catalyst was recorded as Zn-Sn / C. The yield of methyl lactate is shown in Table 1.

[0070] Embodiment 14

[0071] Same as Example 12, except that ferric chloride was replaced by magnesium chloride, and the obtained catalyst was recorded as Zn-Mg / C. The yield of methyl lactate is shown in Table 1.

[0072] Embodiment 15

[0073] Zn / C3 and Fe / C were mechanically blended in a mass ratio of 3:1. 0.04 g of the mechanically blended catalyst and 0.2 g of fructose were added to a high-pressure reactor, and 30 g of methanol was added. The reaction was carried out at 220°C for 60 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0074] Example 16

[0075] Zn / C3 and Sn / C were mechanically blended in a mass ratio of 3:1. 0.04 g of the mechanically blended catalyst and 0.2 g of fructose were added to a high-pressure reactor, and 30 g of methanol was added. The reaction was carried out at 220°C for 60 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0076] Comparative Example 1

[0077] Zinc chloride and water were prepared into zinc chloride molten salt hydrate in a molar ratio of 1:3. 30 g of zinc chloride molten salt hydrate and 0.2 g of glucose were added into a high-pressure reactor, and 30 g of methanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of methyl lactate is shown in Table 1.

[0078] Comparative Example 2

[0079] Zinc chloride and water were prepared into zinc chloride molten salt hydrate in a molar ratio of 1:3. 30 g of zinc chloride molten salt hydrate and 0.2 g of fructose were added into a high-pressure reactor, and 30 g of ethanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of ethyl lactate is shown in Table 1.

[0080] Comparative Example 3

[0081] Zinc chloride and water were prepared into zinc chloride molten salt hydrate in a molar ratio of 1:3. 30 g of zinc chloride molten salt hydrate and 0.2 g of mannose were added into a high-pressure reactor, and 30 g of propanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of propyl lactate is shown in Table 1.

[0082] Comparative Example 4

[0083] Zinc chloride and water were prepared into zinc chloride molten salt hydrate in a molar ratio of 1:3. 30 g of zinc chloride molten salt hydrate and 0.2 g of fructose were added into a high-pressure reactor, and 30 g of n-butanol was added. The reaction was carried out at 220° C. for 60 min in a 2 MPa nitrogen atmosphere. The yield of butyl lactate is shown in Table 1.

[0084] Comparative Example 5

[0085] Referring to Chinese patent CN113831238A, fructose is used as substrate and methanol is used as reaction solvent. 0.2g Sn-SCM-1 molecular sieve, 0.4g fructose and 16g methanol are added to a stirred autoclave. 0.5MPa nitrogen is filled to prevent the organic solvent from boiling. The temperature is raised to the preset temperature by a programmed heating jacket, and then stirred by magnetic stirring. The reaction is carried out at 180°C for 6h, and the reaction solution is separated and analyzed. The yield and selectivity of methyl lactate are shown in Table 1.

[0086] Comparative Example 6

[0087] Referring to Chinese patent CN112062673A, first, 3g of fructose and 300ml of methanol were added to a 500ml high-temperature and high-pressure reactor with a stirrer, and the mass concentration of fructose was 10g / L. Finally, 1.0wt% Ga-1.5wt% Sn / HY catalyst was added, and the catalyst dosage was 0.75g. The valve of the nitrogen cylinder was opened, and the nitrogen pressure was adjusted to 3MPa. After the air in the reactor was discharged, the nitrogen pressure in the reactor was adjusted to 3MPa; then, the temperature of the reactor was adjusted to 140°C by adjusting the temperature of the external heating of the reactor, and the reaction time was 360min; after the reaction was completed, it was cooled to room temperature, filtered, and the filtrate was distilled to obtain methyl lactate product. The yield and selectivity of methyl lactate are shown in Table 1.

[0088] Table 1 Lactic acid ester yield results of Examples 1-16 and Comparative Examples 1-6

[0089]

[0090]

Claims

1. A method for preparing an acid-base carbon-based catalyst, characterized in that The metal salt and water are mixed to form a molten salt hydrate, and biomass is added to the molten salt hydrate for pretreatment; The pretreated biomass is roasted, washed, and dried to obtain an acid-base carbon-based catalyst; The pretreatment is swelling.

2. The method for preparing an acid-base carbon-based catalyst according to claim 1, characterized in that The metal salt is one or more of zinc chloride, zinc bromide, zinc trifluoromethanesulfonate, magnesium chloride, lead chloride, ferric chloride, nickel chloride, stannous chloride, stannic chloride or cobalt chloride, and the molar ratio of the metal salt to water is 1:1-10.

3. The method for preparing the acid-base carbon-based catalyst according to claim 1, characterized in that The biomass is one or more of wheat straw, corn straw or pine sawdust, and the mass of the biomass is 1-20wt.% of the mass of the molten salt hydrate.

4. The method for preparing the acid-base carbon-based catalyst according to claim 1, characterized in that The pretreatment temperature is 30-120° C., and the pretreatment time is 1-12 hours.

5. The method for preparing the acid-base carbon-based catalyst according to claim 1, characterized in that The calcination temperature is 300-500°C, and the calcination time is 3-6h.

6. An application of a catalyst prepared by the method for preparing an acid-base carbon-based catalyst according to any one of claims 1 to 5, characterized in that Acid-base carbon-based catalyst and biomass carbohydrates are added into an alcohol solvent for reaction to obtain lactic acid ester.

7. The use according to claim 6, characterized in that The biomass carbohydrate is one or more of glucose, mannose or fructose, and the alcohol solvent is one of methanol, ethanol, propanol or n-butanol.

8. The use according to claim 6, characterized in that The mass ratio of the acid-base carbon-based catalyst to the biomass carbohydrate is 1:4-6, and the mass ratio of the biomass carbohydrate to the alcohol solvent is 1:140-160.

9. The use according to claim 6, characterized in that The reaction temperature is 160-240° C., and the reaction time is 15-120 min.

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