Method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on la2o3 catalyst

Abstract

The application discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on a La2O3 catalyst, and comprises the following steps: mixing sugar-rich microalgae, a composite catalyst and water to obtain a blend, excluding air, stirring and reacting, cooling, centrifuging, filtering the centrifugal liquid, and obtaining lactic acid. The method is characterized in that the sugar-rich microalgae is cultured by using mixed wastewater obtained by compounding cooling tower wastewater and domestic wastewater, and the lactic acid is prepared by using a composite catalyst for hydrothermal catalysis of the sugar-rich microalgae. The method has the advantages of mild reaction conditions, simple operation, high efficiency, environmental friendliness, high lactic acid yield and great potential application value.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization technology of primary biomass, specifically relating to a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst. Background Technology

[0002] Lactic acid is a versatile and high-potential platform compound, primarily used in pharmaceuticals, the food industry, biodegradable plastics, and as an environmentally friendly solvent. Preparing lactic acid, a small-molecule oxygenated chemical, from biomass via catalytic methods can overcome the problems of long reaction cycles, demanding conditions, and expensive enzymes associated with traditional fermentation methods, showing broad development prospects. However, for terrestrial biomass, the catalytic technology, especially hydrothermal catalysis, mainly utilizes the five- and six-carbon sugars in hemicellulose and cellulose to convert them into lactic acid through bond breaking. However, lignin and ash in native biomass hinder the hydrolysis reaction and have a certain toxic effect on the catalyst; therefore, pretreatment of the native biomass is necessary to improve the selectivity of lactic acid production. Compared to terrestrial biomass, carbohydrate-rich microalgae obtained from nitrogen-deficient cultivation can serve as an ideal raw material for lactic acid production. Due to their lignin-free and low-ash properties, they can be directly collected and concentrated for hydrothermal reaction to produce lactic acid without pretreatment.

[0003] Patent CN 107628938A discloses a method for preparing lactic acid from microalgae using a lanthanum-based catalyst. Microalgae and water are hydrolyzed in the presence of a lanthanum-based catalyst. After the reaction, lactic acid is obtained through treatment. The lanthanum-based catalyst is selected from one or more of La₂O₃, LaCoO₃, and La(OH)₃, but the lactic acid yield is low. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, which solves the technical problems of low lactic acid yield and low catalyst recovery rate in the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on a La2O3 catalyst, wherein the catalyst is a composite catalyst prepared by a method comprising the following steps:

[0007] A1. Al(OH)3 is calcined to obtain γ-Al2O3;

[0008] A2. Dissolve Y(NO3)3·6H2O and La(NO3)3·6H2O in water to obtain a mixed solution, then add the mixed solution dropwise onto the γ-Al2O3 obtained in step A1, stir continuously, dry, grind and calcine to obtain a composite catalyst.

[0009] In step A1, the calcination temperature is 700-900℃, and the calcination time is 2-4 hours; the heating rate during calcination is 3-7℃ / min. This catalyst was obtained through multiple experiments and, within a suitable temperature and time range, can synergistically provide optimal catalytic efficiency with La2O3 and Y2O3. Commercially available catalysts are more difficult to control in terms of calcination temperature and time.

[0010] In steps A1 and A2, the ratio of Al(OH)3, Y(NO3)3·6H2O, La(NO3)3·6H2O, and water is 20-40 kg: 2.45-5 kg: 2.1-4.2 kg: 30-50 L; the dropping rate is 0.1-0.5 mL / s. Y and La are also unique in that they exhibit high selectivity, low-temperature catalysis, and reproducibility in catalyzing the production of lactic acid from polysaccharides.

[0011] In step A2, the mixed solution is dropwise added to solid alumina. By controlling the dropping rate, it gradually diffuses and adsorbs on the alumina surface (La2O3 and Y2O3 diffuse and adsorb onto γ-Al2O3). If alumina is added directly to the mixed solution, it is not conducive to controlling the dropping rate and amount, which is not conducive to controlling the reaction process. It is also not conducive to slow diffusion and adsorption, and the acidity of the final catalyst surface will change, affecting the final catalytic effect. If the three are directly melted and mixed, the calcination temperature is very high. Simple blending is equivalent to blending independent solid catalysts, and the catalytic effect is very poor. However, adding the Y(NO3)3·6H2O and La(NO3)3·6H2O mixed solution dropwise onto γ-Al2O3 and then calcining it can form a compact structure and an appropriate total surface acidity, thereby improving the catalytic effect.

[0012] In step A2, the stirring time is 12-36 hours at room temperature; the drying temperature is 100-120℃ and the time is 12-24 hours.

[0013] In step A2, the calcination temperature is 500-800℃, and the time is 2-4 hours. The two calcined oxides work synergistically to improve the catalytic effect of the catalyst.

[0014] This invention utilizes nitrates, which have high solubility and whose calcination products are nitrogen oxides, making them easy to handle and not polluting the air. Chloride salts, which have relatively better solubility, produce chlorine gas upon calcination, so only nitrates are used here. Al(OH)3, Y(NO3)3·6H2O, and La(NO3)3·6H2O can be calcined at different temperatures to obtain products with good purity. Al(OH)3, Y(NO3)3·6H2O, and La(NO3)3·6H2O cannot be directly mixed and calcined.

[0015] By adopting the above technical solution, the composite catalyst has a high specific surface area, a large total pore volume, and a small pore size, exhibiting a narrow average pore size distribution centered at 9 nm. By modifying γ-Al2O3 with Y and La, new Lewis acid sites and Brønsted acid sites are generated through their interaction (the electronic structure and large empty orbitals of Y and La enable them to form new Lewis acid sites and Brønsted acid sites). The composite catalyst can highly catalyze the conversion of polysaccharides in microalgae and exhibits unique properties and excellent product selectivity for lactic acid.

[0016] This invention provides a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, comprising the following steps:

[0017] A blend was prepared by mixing sugar-rich microalgae, a composite catalyst, and water. The mixture was stirred, cooled, and centrifuged. The centrifuged liquid was then filtered to obtain lactic acid. The composite catalyst was obtained by calcining Al(OH)3, adding a mixed solution containing Y(NO3)3·6H2O and La(NO3)3·6H2O, and then calcining the resulting solution. In this process, the sugar-rich microalgae first hydrolyze into soluble monosaccharides, and then, through the synergistic effect of Lewis acid and Brønsted acid sites, promote the reverse aldol condensation reaction to generate lactic acid.

[0018] Preferably, the ratio of sugar-rich microalgae, composite catalyst, and water is (0.1-0.2) kg: (0.008-0.1) g: (12-24) L.

[0019] Preferably, the air is purged before the blend is stirred and reacted, specifically by purging with helium gas at a pressure of 2.0-4.0 MPa 3-5 times to remove air; the stirring reaction conditions are: temperature 220-260℃, time 20-120 min, and rotation speed 500-700 r / min; the cooling method is to rapidly cool the reactor to room temperature using an ice-water bath; and the filtration method is to filter through a filter membrane with a pore size of 0.20-0.24 μm.

[0020] Preferably, the sugar-rich microalgae are obtained by culturing microalgae in a nitrogen-deficient environment, and the specific culturing method is as follows:

[0021] S1. Add water to dilute the microalgae (thickening microalgae) to obtain pretreated microalgae;

[0022] S2. Inoculate the pretreated microalgae into the culture medium, then add mixed wastewater. Continue to add mixed wastewater daily to maintain the same volume for 7-14 days to obtain sugar-rich microalgae.

[0023] In step S1, the microalgae include at least one of cyanobacteria, green algae, Chlorella, and Chlamydomonas; more preferably, at least one of cyanobacteria and green algae.

[0024] In step S1, the volume ratio of the microalgae (thickening microalgae) to the pretreated microalgae is 0.05-0.1:1.2-2.4. The microalgae (thickening microalgae) undergoes a purification process to remove impurities, specifically by precipitating the collected or commercially available microalgae, removing the supernatant, and the precipitation time is 30-60 minutes.

[0025] In step S2, the culture medium is non-sterile BG-11 medium, and the culture conditions are: temperature 15℃-31℃, light intensity 17500-19500 lux, and light / dark cycle 12h:12h. The culture medium is added to the photobioreactor and is renewed every 7 days.

[0026] In step S2, the volume ratio of pretreated microalgae to pretreated microalgae after adding mixed wastewater is (1.2-2.4):2.5. The mixed wastewater is added at the end of each day's dark cycle, maintaining the total volume as it was on the first day.

[0027] In step S2, the mixed wastewater is a mixed wastewater with a high C / N molar ratio; specifically, it is a mixture of cooling tower wastewater and domestic wastewater with a volume ratio of (65-85):(35-15).

[0028] The wastewater from the cooling tower has a chemical oxygen demand (COD) of 85.5-89.5 mg / L. -1 Total ammonia nitrogen 0.02-0.04 mg / L -1 Nitrate 11.4-12.6 mg N-NO3 - L -1 Nitrite 3.09-3.41 mg N-NO2 - L -1 Phosphate 4.65-4.95 mg P-PO4 3- L -1 pH is 8.2-9.

[0029] The quality of the domestic wastewater is as follows: Chemical Oxygen Demand (COD) is 82.38-86.21 mg O2 / L. -1 Total ammonia nitrogen 77.26-81.2 mg / L -1 Nitrate 34.5-36.2mg N-NO3 - L-1 Nitrite 0.10-0.16mg N-NO2 - L -1 Phosphate 20.8-24.5mg P-PO4 3- L -1 pH is 7.0-7.4.

[0030] By adopting the above technical solution, cooling tower wastewater has the characteristic of high C / N molar ratio, which can naturally increase the carbohydrate content in microalgae without any external carbon source, while degrading organic matter, macronutrients and micronutrients. However, the lack of macronutrients in cooling tower wastewater leads to poor biomass growth and low productivity. By mixing it with domestic wastewater, biomass growth and carbohydrate content can be promoted. The carbohydrate accumulation of sugar-rich microalgae cultured by mixing wastewater is significantly increased, and large flocs are produced during the growth of microalgae, which enhances natural sedimentation and facilitates biomass harvesting.

[0031] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0032] (1) The present invention cultivates microalgae by mixing cooling tower wastewater and domestic wastewater, which not only increases the carbohydrate content of microalgae to about 50-60%, but also solves the problem of wastewater treatment and reuse, which is simple and environmentally friendly. At the same time, the sugar-rich microalgae are easy to obtain and grow rapidly, and have the characteristics of not containing lignin and having low ash content. Therefore, lactic acid can be prepared by hydrothermal reaction after simple collection and concentration without pretreatment.

[0033] (2) The present invention generates new Lewis acid sites and Brønsted acid sites through the interaction of Y, La and γ-Al2O3. Microalgae can be depolymerized into glucose through CO bond cleavage catalyzed by Brønsted acid sites. Moreover, Lewis acid sites can coordinate with the carbonyl group of soluble oligosaccharides and promote the selective cleavage condensation reaction of C3-C4 bond through reverse aldol to generate triose C intermediate. Strong Lewis acid sites are conducive to the reverse aldol condensation of carbohydrates, while weak Lewis acid sites easily promote the isomerization of carbohydrates. The Y2O3 surface has dominant medium acid sites and fewer strong acid sites. The active sites of Y and La have a synergistic effect with γ-Al2O3, which significantly improves the catalytic activity and selectivity of microalgae to produce lactic acid. The composite catalyst is easy to separate and can be reused. The present invention’s “one-step” hydrothermal catalytic preparation of lactic acid from sugar-rich microalgae has mild reaction conditions, simple operation, economic efficiency, and is environmentally friendly, with great potential application value. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment discloses a method for cultivating sugar-rich microalgae, including the following steps:

[0037] S1: Collected microalgae (prepared from cyanobacteria, green algae, Chlorella, and Chlamydomonas separately) were precipitated in an Imhoff sedimentation conical flask for 45 min. After removing the supernatant, 0.075 L of thickened microalgae was obtained. Water was then added to dilute the microalgae to obtain 1.8 L of pretreated microalgae.

[0038] S2: Add non-sterile BG-11 medium to the photobioreactor and refresh it every 7 days. Under the conditions of temperature 25℃, light intensity 18500 lux, and light / dark cycle of 12h:12h, 1.8L of pretreated microalgae is inoculated into the photobioreactor. Then, cooling tower wastewater and domestic wastewater are added to the photobioreactor at a volume ratio of 75:25 v / v to a volume of 2.5L. At the end of the dark cycle each day, mixed wastewater is added to maintain the total volume. This process is continued for 10 days to obtain sugar-rich microalgae (cyanobacteria, green algae, Chlorella, Chlamydomonas).

[0039] The wastewater quality of the cooling tower is as follows: Chemical Oxygen Demand (COD) is 87.5 mg / L. -1 Total ammonia nitrogen 0.03 mg / L -1 12.4 mg of nitrate N-NO3 - L -1 Nitrite 3.29mg N-NO2 - L -1 Phosphate 4.85mg P-PO4 3- L -1 The pH value is 8.6; the chemical oxygen demand (COD) of the domestic wastewater is 85.377 mg / L. -1 Total ammonia nitrogen 79.26 mg / L, nitrate 35.5 mg N-NO3 - L -1 Nitrite 0.13mg N-NO2 - L -1 Phosphate 22.8mg P-PO4 3- L -1 The pH value is 7.23.

[0040] Example 2

[0041] This embodiment discloses a method for preparing a composite catalyst, including the following steps:

[0042] A1: Under air atmosphere, 30 kg of Al(OH)3 was heated to 800 °C at a heating rate of 5 °C / min and then calcined for 3 h to obtain γ-Al2O3.

[0043] A2: Dissolve 3.75 kg Y(NO3)3·6H2O and 3.1 kg La(NO3)3·6H2O in 40 L of water to obtain a mixed solution. Then, add the mixed solution dropwise onto γ-Al2O3 at a rate of 0.3 mL / s. Stir continuously at room temperature for 24 h, dry at 110 °C for 18 h, grind, and calcine at 650 °C for 3 h to obtain a composite catalyst.

[0044] Example 3

[0045] This embodiment discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, including the following steps:

[0046] 0.15 kg of sugar-rich cyanobacteria cultured in Example 1, 0.02 kg of the composite catalyst prepared in Example 2, and 18 L of deionized water were mixed to obtain a blend. After purging with 3 MPa helium gas four times to remove air, the mixture was stirred at 600 r / min for 50 min at 240 °C. After the reaction was completed, the reactor was rapidly cooled to room temperature using an ice-water bath. The mixture was centrifuged, and the centrifuged liquid was filtered through a 0.22 μm pore size filter membrane to obtain lactic acid.

[0047] Example 4

[0048] This embodiment discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, including the following steps:

[0049] 0.1 kg of sugar-rich green algae cultured in Example 1, 0.008 kg of the composite catalyst prepared in Example 2, and 24 L of deionized water were mixed to obtain a blend. After purging with 2.0 MPa helium five times to remove air, the mixture was stirred at 700 r / min for 120 min at 220 °C. After the reaction was completed, the reactor was rapidly cooled to room temperature using an ice-water bath. The liquid product was centrifuged and filtered through a 0.20 μm pore size membrane to obtain lactic acid.

[0050] Example 5

[0051] This embodiment discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, including the following steps:

[0052] 0.2 kg of sugar-rich Chlorella cultured in Example 1, 0.1 kg of the composite catalyst prepared in Example 2, and 12 L of deionized water were mixed to obtain a blend. After purging with 4 MPa helium gas three times to remove air, the mixture was stirred at 500 r / min for 20 min at 260 °C. After the reaction was completed, the reactor was rapidly cooled to room temperature using an ice-water bath. The mixture was centrifuged, and the centrifuged liquid was filtered through a 0.20 μm pore size filter membrane to obtain lactic acid.

[0053] Example 6

[0054] This embodiment discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, including the following steps:

[0055] 0.11 kg of *Chlamydomonas sacchariformis* cultured in Example 1, 0.08 kg of the composite catalyst prepared in Example 2, and 15 L of deionized water were mixed to obtain a blend. After purging with 2.6 MPa helium gas three times to remove air, the mixture was stirred at 550 r / min for 40 min at 230 °C. After the reaction was completed, the reactor was rapidly cooled to room temperature using an ice-water bath. The mixture was centrifuged, and the centrifuged liquid was filtered through a 0.21 μm pore size membrane to obtain lactic acid.

[0056] Example 7

[0057] This embodiment discloses a method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, including the following steps:

[0058] 0.16 kg of sugar-rich cyanobacteria cultured in Example 1, 0.05 kg of the composite catalyst prepared in Example 2, and 22 L of deionized water were mixed to obtain a blend. After purging with 3.2 MPa helium gas five times to remove air, the mixture was stirred at 550 r / min at 240 °C for 80 min. After the reaction was completed, the reactor was rapidly cooled to room temperature using an ice-water bath. The mixture was centrifuged, and the centrifuged liquid was filtered through a 0.23 μm pore size filter membrane to obtain lactic acid.

[0059] Comparative Example 1

[0060] Compared with Example 3, Comparative Example 1 used an equal amount of La2O3 to replace the composite catalyst in the process of preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, while keeping other conditions unchanged.

[0061] Comparative Example 2

[0062] Compared with Example 3, Comparative Example 2 used Y2O3 to replace the composite catalyst in the process of preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, while keeping other conditions unchanged.

[0063] Comparative Example 3

[0064] Compared with Example 3, Comparative Example 3 used an equal amount of ordinary cyanobacteria to replace sugar-rich cyanobacteria in the process of preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, while keeping other conditions unchanged.

[0065] Comparative Example 4

[0066] Compared with Example 3, Comparative Example 4 used 0.002g La2O3, 0.002g Y2O3 and 0.016g Al2O3 to mix and stir during the preparation of lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, replacing 0.02kg composite catalyst in Example 3 in the same amount, while keeping other conditions unchanged.

[0067] Comparative Example 5

[0068] Compared with Example 3, in the process of preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, the composite catalyst in Comparative Example 5 was prepared by dissolving 3.75 kg Y(NO3)3·6H2O and 3.1 kg La(NO3)3·6H2O in 40 L of water to obtain a mixed solution, without using the dropwise addition method, and directly mixing it with γ-Al2O3, while keeping other conditions unchanged.

[0069] The yields of lactic acid prepared by hydrolysis of sugar-rich microalgae based on La2O3 catalyst in Examples 3-7 and Comparative Examples 1-3 were tested.

[0070] High-performance liquid chromatography (HPLC) (Shimadzu LC-20AD, Aminex HPX-87H, Ion Exclusion Column: 300 mm × 7.8 mm, RID-10A) was used for quantitative analysis of the liquid products. The analytical conditions were as follows: column temperature 50℃, flow rate set at 0.65 mL / min, and mobile phase selected as 12.5 mmol / L H₂SO₄ solution. Before quantitative analysis of the samples, a standard curve was plotted using standards for the analytes. Then, under the same chromatographic conditions, the component content in the liquid products was measured, and the concentration was analyzed based on the standard curve.

[0071] Lactic acid yield (%) = (total C moles in the product lactic acid / total C moles in the raw material) × 100%.

[0072] Carbon yield is calculated as the ratio of total carbon in the liquid product to the carbon content in the feedstock, multiplied by 100%.

[0073] Table 1

[0074]

[0075] As shown in Table 1, the methods for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst in Examples 3-7 of this invention have good carbon and lactic acid yields. A comparison between Comparative Examples 1-5 and Examples 3-7 shows that the composite catalyst improves carbon and lactic acid yields more than the single catalyst. A comparison between Comparative Example 3 and Examples 3-7 shows that sugar-rich microalgae have higher carbon and lactic acid yields than ordinary microalgae. A comparison between Comparative Example 4 and Examples 3-7 shows that using the composite catalyst results in higher carbon and lactic acid yields than mixing and stirring La2O3, Y2O3, and Al2O3. A comparison between Comparative Example 5 and Examples 3-7 shows that the composite catalyst prepared by dropwise addition has higher carbon and lactic acid yields.

[0076] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0077] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing lactic acid by hydrolysis of sugar-rich microalgae based on La2O3 catalyst, characterized in that, The catalyst used in this method is a composite catalyst prepared by the following steps: A1. Al(OH)3 is calcined to obtain γ-Al2O3; A2. Dissolve Y(NO3)3·6H2O and La(NO3)3·6H2O in water to obtain a mixed solution. Then add the mixed solution dropwise onto the γ-Al2O3 obtained in step A1, stir continuously, dry, grind and calcine to obtain a composite catalyst. In step A1, the calcination temperature is 700-900 ℃, the calcination time is 2-4 h, and the heating rate during calcination is 3-7 ℃ / min. In steps A1 and A2, the ratio of Al(OH)3, Y(NO3)3·6H2O, La(NO3)3·6H2O, and water is 20-40 kg: 2.45-5 kg: 2.1-4.2 kg: 30-50 L; the dropping rate is 0.1-0.5 mL / s. In step A2, the calcination temperature is 500-800℃ and the time is 2-4 hours; The method for preparing lactic acid by hydrolysis of sugar-rich microalgae includes the following steps: mixing sugar-rich microalgae, composite catalyst and water to obtain a blend, stirring to react, cooling and centrifuging, and filtering the centrifuged liquid to obtain lactic acid; the sugar-rich microalgae are obtained by culturing microalgae in a nitrogen-deficient environment.

2. The method for preparing lactic acid by hydrolysis of sugar-rich microalgae according to claim 1, characterized in that, The ratio of sugar-rich microalgae, composite catalyst, and water is 0.1-0.2 kg: 0.008-0.1 g: 12-24 L.

3. The method for preparing lactic acid by hydrolysis of sugar-rich microalgae according to claim 1, characterized in that, The specific cultivation method for the nitrogen-deficient culture is as follows: S1. Dilute the microalgae with water to obtain pretreated microalgae; S2. Inoculate the pretreated microalgae into the culture medium, then add mixed wastewater. Continue to add mixed wastewater daily to maintain a constant volume for 7-14 days to obtain sugar-rich microalgae. The mixed wastewater is obtained by combining cooling tower wastewater and domestic wastewater.

4. The method for preparing lactic acid by hydrolysis of sugar-rich microalgae according to claim 3, characterized in that, In step S1, the microalgae include at least one of cyanobacteria, green algae, Chlorella, and Chlamydomonas.

5. The method for preparing lactic acid by hydrolysis of sugar-rich microalgae according to claim 3, characterized in that, In step S2, the culture medium is BG-11 medium, and the culture conditions are: temperature 15°C-31°C, light intensity 17500-19500 lux, and light / dark cycle 12h:12h.

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