CN115779981B - Bamboo charcoal-based methyl lactate catalyst and preparation method and application thereof - Google Patents

Abstract

The invention provides a preparation method of a bamboo charcoal-based methyl lactate catalyst, which comprises the steps of impregnating bamboo chips into SnCl ₄ According to the method, tetravalent tin ions are fully immersed into an internal bamboo vascular bundle of the bamboo chips, then the bamboo vascular bundle is carbonized through phased temperature programming, tin-based ions are efficiently and uniformly introduced into a carbon skeleton, phased temperature programming is performed, phased temperature-increasing rate, temperature-keeping temperature and temperature-keeping time are cooperatively controlled, the specific surface area of a carbon material is increased, the micro-pore is promoted to be formed, the catalysis efficiency is improved, meanwhile, high Wen Cushi tin-based ions are involved in the modeling of carbon-based holes, high-temperature loss is avoided, the combination of the tin-based ions and active sites in the holes is firmer, the problem that catalytic metal elements are lost in the using process is solved, and finally the bamboo carbon-based methyl lactate catalyst with high and stable catalysis efficiency and long service life is obtained.

Description

A bamboo charcoal-based methyl lactate catalyst and its preparation method and application

Technical field

The invention relates to the technical field of catalytic materials, and in particular to a bamboo charcoal-based methyl lactate catalyst and its preparation method and application.

Background technique

Plastics are made from non-renewable petrochemical products. In 2021, nine departments jointly issued the "Notice on Solidly Promoting Plastic Pollution Control", banning the use of non-degradable plastic shopping bags. Therefore, using biomass materials to replace petrochemical products has become a mainstream trend, and lactic acid products are an ideal substitute for traditional plastic materials. At present, there are two main ways to obtain lactic acid, one is biological fermentation and the other is chemical preparation. Among them, biological fermentation has the characteristics of low requirements for raw materials and high lactic acid conversion rate, and is a widely used method at this stage. However, during use, it was also discovered that the fermentation method has the disadvantages of slow enzymatic reaction rate, low space-time yield, high energy consumption, and difficult purification of raw materials. Therefore, the chemical preparation method of producing lactic acid by catalyzing the easy-to-obtain simple substrate glucose under heterogeneous hydrothermal conditions has become an important and effective way, and the heterogeneous catalyst in the chemical preparation method is the key to the entire system.

Heterogeneous catalysts have the advantages of being easy to separate from products, recyclable, and non-corrosive to equipment. However, the current preparation process of lactic acid heterogeneous catalysts is relatively backward, and the synthesis process of supporting materials such as B molecular sieve is complex and its commercial products are expensive. In terms of research on the use of biomass carbon as a catalyst carrier, the carbonized product is mainly made by loading catalytic metal through its micro-mesopores and using a high-temperature calcining furnace. There are problems of serious ablation and easy loss of the loaded catalytic metal, which not only affects the use of the catalyst efficiency and shorten the service life of the catalyst. Therefore, there is an urgent need for a biomass-based lactic acid heterogeneous catalyst with high catalytic efficiency, stability, and long service life.

Contents of the invention

The object of the present invention is to provide a bamboo charcoal-based methyl lactate catalyst and its preparation method and application. The bamboo charcoal-based methyl lactate catalyst prepared by the method provided by the invention can be used as a lactic acid heterogeneous catalyst and has high catalytic efficiency and stability. Advantages of long service life.

In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

The invention provides a method for preparing a bamboo charcoal-based methyl lactate catalyst, which includes the following steps:

(1) Dip bamboo chips into an aqueous solution of SnCl ₄ to obtain impregnated bamboo chips;

(2) Carbonize the impregnated bamboo chips obtained in step (1) to obtain a bamboo charcoal-based methyl lactate catalyst;

The staged programmed temperature rise method of the carbonization treatment is: in the first stage, the temperature is raised from room temperature to 150~200°C at a rate of 4~12°C/min and kept for 28~65min; in the second stage, the temperature is increased from 150~200°C at a rate of 4~12°C/min. Raise the temperature to 270-380°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; in the third stage, raise the temperature from 270-380°C to 520-600°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; In the fourth stage, the temperature is raised from 520 to 600°C at a rate of 4 to 12°C/min to 720 to 800°C and kept for 28 to 65 minutes; in the fifth stage, the temperature is raised from 720 to 800°C at a rate of 4 to 12°C/min. 880～920℃, keep warm for 50～75min.

Preferably, in the step (1), the length of the bamboo piece is 3-10cm, the width of the bamboo piece is 2-6cm, and the thickness of the bamboo piece is 0.3-1.7cm.

Preferably, the aqueous solution of SnCl ₄ in step (1) consists of SnCl ₄ ·5H ₂ O and water with a mass ratio of 1: (3-8).

Preferably, the soaking time in step (1) is 2 to 10 hours.

Preferably, the soaking time is 3 to 9 hours.

Preferably, after the impregnation is completed in step (1), it further includes: draining the impregnated product naturally.

Preferably, the carbonization treatment in step (2) is performed in an inert atmosphere.

Preferably, the step-by-step programmed temperature rise method of the carbonization treatment in step (2) is: in the first stage, the temperature is raised from room temperature to 160-190°C at a rate of 5-10°C/min, and kept warm for 30-60 minutes; in the second step, In the first stage, the temperature is raised from 160 to 190°C to 280 to 350°C at a rate of 5 to 10°C/min, and kept warm for 30 to 60 minutes; in the third stage, the temperature is raised from 280 to 350°C to 530 to 530°C at a rate of 5 to 10°C/min. 580℃, keep warm for 30~60min; the fourth stage, heat from 530~580℃ at a rate of 5~10℃/min to 730~780℃, keep warm for 30~60min; the fifth stage, heat from 730~780℃ at a rate of 5~10℃/min. Raise the temperature to 890~910℃ at a rate of 10℃/min and keep it warm for 55~70min.

The present invention also provides a bamboo charcoal-based methyl lactate catalyst prepared by the preparation method described in the above technical solution.

The present invention also provides the bamboo charcoal-based methyl lactate catalyst prepared by the preparation method of the above technical solution or the application of the bamboo charcoal-based methyl lactate catalyst in the preparation of lactic acid heterogeneous catalyst.

The invention provides a method for preparing a bamboo charcoal-based methyl lactate catalyst. Bamboo slices are immersed in an aqueous solution of SnCl ₄ to obtain impregnated bamboo slices, so that tetravalent tin ions are fully impregnated into the internal bamboo vascular bundles of the bamboo slices. , and then perform carbonization treatment by programmed temperature rise in stages, and introduce tin-based ions into the carbon skeleton efficiently and uniformly. Through staged program temperature rise, the staged heating rate, holding temperature and holding time are collaboratively controlled to achieve the removal of bamboo in the first stage. The flakes are free water in the bamboo. In the second stage, the cracking of bamboo hemicellulose and the removal of bound water are achieved. In the third stage, the cracking of bamboo lignin and cellulose is achieved. In the fourth stage, the cracking of bamboo hemicellulose and the removal of bound water are achieved. The bamboo charcoal material is carbonized to open pores. In the fifth stage, the specific surface area of the bamboo charcoal material is further increased to promote the formation of micropores, which improves the catalytic efficiency of the prepared catalyst. At the same time, the high temperature promotes tin-based ions to participate in the carbon-based pore shape, which avoids The high temperature loss makes the tin-based ions more firmly combined with the active sites in the pores to overcome the problem of the loss of catalytic metal elements during use. Finally, a bamboo charcoal-based methyl lactate catalyst with high catalytic efficiency, stability, and long service life is obtained. The method provided by the invention is a purely physical process with no chemical residues. The results of the examples show that the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in the examples of the present invention can reach 802.0367m ² /g. As a lactic acid heterogeneous catalyst, the yield of catalyzing glucose to prepare methyl lactate is more than 42%. And after being reused ten times, its catalytic efficiency still remains above 95%.

Description of the drawings

Figure 1 isothermal adsorption and desorption curve of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 of the present invention;

Figure 2 is a gas ion chromatogram of the product obtained after the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 of the present invention catalyzes the conversion of glucose;

Figure 3 is an isothermal adsorption-desorption curve of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 of the present invention;

Figure 4 is a gas ion chromatogram of the product obtained after the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 of the present invention catalyzes the conversion of glucose.

Detailed ways

The invention provides a method for preparing a bamboo charcoal-based methyl lactate catalyst, which includes the following steps:

(1) Dip bamboo chips into an aqueous solution of SnCl ₄ to obtain impregnated bamboo chips;

(2) Carbonize the impregnated bamboo chips obtained in step (1) to obtain a bamboo charcoal-based methyl lactate catalyst;

The staged programmed temperature rise method of the carbonization treatment is: in the first stage, the temperature is raised from room temperature to 150~200°C at a rate of 4~12°C/min and kept for 28~65min; in the second stage, the temperature is increased from 150~200°C at a rate of 4~12°C/min. Raise the temperature to 270-380°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; in the third stage, raise the temperature from 270-380°C to 520-600°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; In the fourth stage, the temperature is raised from 520 to 600°C at a rate of 4 to 12°C/min to 720 to 800°C and kept for 28 to 65 minutes; in the fifth stage, the temperature is raised from 720 to 800°C at a rate of 4 to 12°C/min. 880～920℃, keep warm for 50～75min.

In the present invention, unless otherwise specified, the raw materials used are all conventional commercial products in this field.

In the present invention, bamboo chips are immersed in an aqueous solution of SnCl ₄ to obtain impregnated bamboo chips.

In the present invention, the length of the bamboo piece is preferably 3-10cm, more preferably 4-8cm; the width of the bamboo piece is preferably 2-6cm, more preferably 3-5cm; the thickness of the bamboo piece is preferably It is 0.3-1.7cm, and it is more preferable that it is 0.5-1.5cm. By controlling the length, width and thickness of the bamboo slices, the present invention promotes the tetravalent tin ions in the SnCl ₄ aqueous solution to fully enter the internal bamboo vascular bundles of the bamboo slices.

In the present invention, the aqueous solution of SnCl ₄ is preferably composed of SnCl ₄ ·5H 2 O and water with a mass ratio of 1: (3-8), and more preferably is composed of SnCl 4 ·5H ₂ O and water with a mass ratio of 1: (4-6) Composed of SnCl ₄ ·5H ₂ O and water. The present invention controls the mass ratio of _{SnCl 4 ·} 5H ₂ O and water in the aqueous solution of SnCl 4 to avoid the hydrolysis of SnCl ₄ to form insoluble matter and affect the impregnation effect.

In the present invention, the impregnation time is preferably 2 to 10 hours, more preferably 3 to 9 hours. By controlling the impregnation time, the present invention promotes the full impregnation of tetravalent tin ions into the interior of the bamboo.

After the impregnation is completed, the present invention preferably drains the impregnated product naturally to obtain impregnated bamboo chips.

The present invention has no special limitation on the natural draining method, as long as the water is removed.

After obtaining the impregnated bamboo chips, the present invention carbonizes the impregnated bamboo chips to obtain a bamboo charcoal-based methyl lactate catalyst.

In the present invention, the carbonization treatment is preferably performed in an inert atmosphere. In the present invention, the inert atmosphere is preferably nitrogen.

In the present invention, the staged programmed temperature rise method of the carbonization treatment is: in the first stage, the temperature is raised from room temperature to 150~200°C at a rate of 4~12°C/min and kept for 28~65min; in the second stage, Raise the temperature from 150 to 200°C to 270 to 380°C at a rate of 4 to 12°C/min, and keep it warm for 28 to 65 minutes; in the third stage, heat from 270 to 380°C to 520 to 600°C at a rate of 4 to 12°C/min. Keep the temperature for 28~65min; the fourth stage, raise the temperature from 520~600℃ to 720~800℃ at a rate of 4~12℃/min, and keep it for 28~65min; the fifth stage, raise the temperature from 720~800℃ at a rate of 4~12℃/min. The temperature is raised to 880~920℃ at a rate of 5~10℃/min and kept for 50~75min; the preferred method is: in the first stage, the temperature is raised from room temperature to 160~190℃ at a speed of 5~10℃/min and kept for 30~60min; in the second stage, the temperature is increased by Raise the temperature from 160 to 190°C to 280 to 350°C at a rate of 5 to 10°C/min, and keep it warm for 30 to 60 minutes; in the third stage, heat from 280 to 350°C to 530 to 580°C at a rate of 5 to 10°C/min. Keep the temperature for 30~60min; the fourth stage, raise the temperature from 530~580℃ to 730~780℃ at a rate of 5~10℃/min, and keep it warm for 30~60min; the fifth stage, raise the temperature from 730~780℃ at a rate of 5~10℃/min. Raise the temperature to 890~910℃ at a rate of 5 min and keep it warm for 55~70 min.

The present invention performs carbonization treatment through staged programmed temperature rise, and collaboratively controls the staged heating rate, heat preservation temperature and heat preservation time. In the first stage, the free water in the bamboo pieces, that is, the bamboo material is removed, and in the second stage, the bamboo material is semi-cooled. In the third stage, the cracking of cellulose and the removal of bound water are achieved. In the third stage, the cracking of bamboo lignin and cellulose is achieved. In the fourth stage, the carbonization and opening of bamboo charcoal materials are achieved. In the fifth stage, the charcoal material is further improved. The specific surface area promotes the formation of micropores and improves the catalytic efficiency of the prepared catalyst. At the same time, high temperature promotes tin-based ions to participate in the carbon-based pore shape, avoiding high-temperature loss and making the tin-based ions more firmly bonded to the active sites in the pores. , in order to overcome the problem of loss of catalytic metal elements during use, and finally obtain a bamboo charcoal-based methyl lactate catalyst with high catalytic efficiency, stability and long service life.

After the carbonization treatment, the present invention preferably cools the carbonized product naturally to obtain a bamboo charcoal-based methyl lactate catalyst.

The present invention has no special limitation on the method of self-heating cooling, and it is sufficient to adopt technical solutions commonly used in this field to achieve cooling to room temperature.

The method provided by the invention is a purely physical process with no chemical residue. The bamboo charcoal-based methyl lactate catalyst prepared by the method provided by the invention can be used as a heterogeneous lactic acid catalyst and has the advantages of high catalytic efficiency, stability, and long service life.

The present invention also provides a bamboo charcoal-based methyl lactate catalyst prepared by the preparation method described in the above technical solution. In the present invention, the bamboo charcoal-based methyl lactate catalyst includes bamboo charcoal and a tin base supported on the bamboo charcoal.

The present invention also provides the bamboo charcoal-based methyl lactate catalyst prepared by the preparation method of the above technical solution or the application of the bamboo charcoal-based methyl lactate catalyst in the preparation of lactic acid heterogeneous catalyst.

The technical solutions in 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 of the embodiments of the present invention, but not 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 fall within the scope of protection of the present invention.

Example 1

(1) Submerge the bamboo slices into an aqueous solution of SnCl ₄ for 3 hours to obtain impregnated bamboo slices;

The aqueous solution of SnCl ₄ is made by fully mixing 20g of analytically pure SnCl ₄ ·5H ₂ O and 100g of water for 30 minutes;

The bamboo pieces are made of bamboo material with a length of 10cm, a width of 3cm, and a thickness of 1.2cm after removing the knots;

(2) Carbonize the impregnated bamboo chips obtained in step (1) to obtain a bamboo charcoal-based methyl lactate catalyst;

The step-by-step programmed temperature rise method of the carbonization treatment is as follows: in the first stage, the temperature is raised from room temperature to 170°C at a rate of 5°C/min, and the temperature is kept for 40 minutes; in the second stage, the temperature is raised from 170°C to 300°C at a rate of 5°C/min, and the temperature is kept. 40min; in the third stage, the temperature is raised from 300℃ to 550℃ at 5℃/min and kept for 40min; in the fourth stage, the temperature is raised from 550℃ to 750℃ at 5℃/min and kept for 30min; in the fifth stage, the temperature is raised at 5℃/min. min. Raise the temperature from 750℃ to 900℃ and keep it warm for 60min.

Using a specific surface area measuring instrument (American ASAP2020), the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 was detected through the nitrogen isothermal adsorption and desorption method, which was 802.0367 m ² /g.

Performance Testing:

1. Adsorption performance test of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1

Using ASAP2020 specific surface area measuring instrument, the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 was degassed at 300°C for 300 minutes, at a liquid nitrogen saturation temperature of 77.4K, using nitrogen (purity: 99.99%) as the adsorption medium, at a relative pressure P /P0 (P, P0 are the equilibrium pressure and saturation pressure of nitrogen low-temperature adsorption respectively) is between 0.01 and 0.995. Measure isothermal adsorption and desorption, and obtain the adsorption and desorption isotherm diagrams as shown in Figure 1;

As can be seen from Figure 1, the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 is ^802.0367m2 /g. The adsorption and desorption curve is closed, indicating that its desorption performance is excellent.

2. Catalytic performance test of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1

Take 2.5g of the bamboo charcoal-based methyl lactate catalyst prepared in Example 1, add it to 20 mL of a methanol solution of glucose with a glucose mass concentration of 2%, place it in a reaction kettle, and react at 160°C for 6 hours. After detecting lactic acid The yield of methyl ester is 42%;

Calculation method: Yield of methyl lactate = (molar mass of lactic acid/208) × 0.5 × 100%;

Detect the mass ion chromatogram of the product obtained after the catalyst prepared in Example 1 catalyzes the above-mentioned glucose conversion according to the following conditions: select a DB-5MS chromatographic column, a column oven of 35°C, an injection volume of 1uL, and a programmed temperature rise of 35°C for insulation. 3 min, raised to 65°C at 2°C/min, raised to 300°C at 5°C/min, and kept for 5 minutes. The mass spectrometer used a Gileen 5975c tripole analyzer, and the spectrum search library was NIST08.L. The preparation of Example 1 was obtained. The gas ion chromatogram of the product obtained after the catalyst catalyzes the conversion of glucose is shown in Figure 2;

As can be seen from Figure 2, the methyl lactate component in the product obtained after the catalyst prepared in Example 1 catalyzed the conversion of glucose accounted for 72.18% of the detected components, indicating that the catalyst prepared in Example 1 has high specificity.

After the above reaction is completed, the bamboo charcoal-based methyl lactate catalyst in the reactor is recovered, the above reaction is carried out according to the same method, and the catalytic performance test is performed. The study found that the bamboo charcoal-based methyl lactate catalyst prepared in Example 1 was reused ten times. Afterwards, its catalytic efficiency remained above 95%.

Example 2

(1) Submerge the bamboo slices into an aqueous solution of SnCl ₄ for 3 hours to obtain impregnated bamboo slices;

The aqueous solution of SnCl ₄ is made by fully mixing 20g of analytically pure SnCl ₄ ·5H ₂ O and 120g of water for 30 minutes;

The bamboo slices are bamboo slices with a length of 10cm, a width of 3cm, and a thickness of 1.2cm after removing the knots;

(2) Carbonize the impregnated bamboo chips obtained in step (1) to obtain a bamboo charcoal-based methyl lactate catalyst;

The step-by-step programmed temperature rise method of the carbonization treatment is: in the first stage, the temperature is raised from room temperature to 190°C at 8°C/min, and the temperature is kept for 30 minutes; in the second stage, the temperature is raised from 190°C to 350°C at 8°C/min, and the temperature is kept. 30min; in the third stage, the temperature is raised from 350℃ to 570℃ at 8℃/min and kept for 30min; in the fourth stage, the temperature is raised from 570℃ to 760℃ at 8℃/min and kept for 30min; in the fifth stage, the temperature is raised at 10℃/min. min. Raise the temperature from 760℃ to 900℃ and keep it warm for 70min.

Using a specific surface area measuring instrument (American ASAP2020), the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 was determined to be 770.9310 m ² /g through the nitrogen isothermal adsorption and desorption method.

1. Adsorption performance test of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2

Using ASAP2020 specific surface area measuring instrument, the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 was degassed at 300°C for 300 minutes, at a liquid nitrogen saturation temperature of 77.4K, using nitrogen (purity: 99.99%) as the adsorption medium, at a relative pressure P /P0 (P, P0 are the equilibrium pressure and saturation pressure of nitrogen low-temperature adsorption respectively) is between 0.01 and 0.995. Measure isothermal adsorption and desorption, and obtain the adsorption and desorption isotherm diagrams as shown in Figure 3;

As can be seen from Figure 3, the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 is ^770.9312m2 /g, and the adsorption and desorption curve is closed, indicating that its desorption performance is excellent.

2. Catalytic performance test of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2

Take 2.5g of the bamboo charcoal-based methyl lactate catalyst prepared in Example 2, add it to 20 mL of a glucose methanol solution with a glucose mass concentration of 2%, place it in a reaction kettle, and react at 175°C for 6 hours. After detection The yield of methyl lactate is 42%;

Calculation method: Yield of methyl lactate = (molar mass of lactic acid/208) × 0.5 × 100%;

Detect the mass ion chromatogram of the product obtained after the catalyst prepared in Example 2 catalyzes the above-mentioned glucose conversion according to the following conditions: select a Jielen DB-5MS chromatographic column, a column temperature of 35°C, an injection volume of 1uL, and a programmed temperature rise of 35 and incubation for 3 minutes. , raised to 65°C at 2°C/min, raised to 300°C at 5°C/min, and kept for 5 minutes. The mass spectrometer was a Gileen 5975c tripole analyzer, and the spectrum search library was NIST08.L. The product prepared in Example 2 was obtained. The gas ion chromatogram of the product obtained after the catalyst catalyzes the conversion of glucose is shown in Figure 2;

As can be seen from Figure 4, the methyl lactate component in the product obtained after the catalyst prepared in Example 2 catalyzed the conversion of glucose accounted for 75.32% of the detected components, indicating that the catalyst prepared in Example 2 has excellent catalytic efficiency.

After the above reaction is completed, the bamboo charcoal-based methyl lactate catalyst in the reactor is recovered, the above reaction is carried out according to the same method, and the catalytic performance test is performed. The study found that the bamboo charcoal-based methyl lactate catalyst prepared in Example 2 was reused ten times. Afterwards, its catalytic efficiency remained above 95%.

In summary, it can be seen that the specific surface area of the bamboo charcoal-based methyl lactate catalyst prepared in the embodiment of the present invention can reach 802.0367m ² /g. As a lactic acid heterogeneous catalyst, the yield of catalyzing glucose to prepare methyl lactate is more than 42%, and repeated After ten uses, its catalytic efficiency remains above 95%.

The above are only preferred embodiments of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications can also be made. should be regarded as the protection scope of the present invention.

Claims (10)

1. A preparation method of bamboo charcoal-based methyl lactate catalyst, comprising the following steps: (1) Dip bamboo chips into an aqueous solution of SnCl ₄ to obtain impregnated bamboo chips; (2) Carbonize the impregnated bamboo chips obtained in step (1) to obtain a bamboo charcoal-based methyl lactate catalyst; The staged programmed temperature rise method of the carbonization treatment is: in the first stage, the temperature is raised from room temperature to 150~200°C at a rate of 4~12°C/min and kept for 28~65min; in the second stage, the temperature is increased from 150~200°C at a rate of 4~12°C/min. Raise the temperature to 270-380°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; in the third stage, raise the temperature from 270-380°C to 520-600°C at a rate of 4-12°C/min and keep it warm for 28-65 minutes; In the fourth stage, the temperature is raised from 520 to 600°C at a rate of 4 to 12°C/min to 720 to 800°C and kept for 28 to 65 minutes; in the fifth stage, the temperature is raised from 720 to 800°C at a rate of 4 to 12°C/min. 880～920℃, keep warm for 50～75min. 2. The preparation method according to claim 1, characterized in that in the step (1), the length of the bamboo slices is 3-10cm, the width of the bamboo slices is 2-6cm, and the thickness of the bamboo slices is 0.3-1.7cm. . 3. The preparation method according to claim 1, characterized in that in the step (1), the aqueous solution of SnCl ₄ is composed of SnCl ₄ ·5H ₂ O and water with a mass ratio of 1: (3-8). 4. The preparation method according to claim 1, characterized in that the impregnation time in step (1) is 2 to 10 hours. 5. The preparation method according to claim 4, characterized in that the impregnation time is 3 to 9 hours. 6. The preparation method according to claim 1, characterized in that after the impregnation is completed in step (1), it further includes: draining the impregnated product naturally. 7. The preparation method according to claim 1, characterized in that in the step (2), the carbonization treatment is carried out in an inert atmosphere. 8. The preparation method according to claim 1, characterized in that the step-by-step programmed temperature rise of the carbonization treatment in step (2) is: in the first stage, the temperature is raised from room temperature at a rate of 5 to 10°C/min. to 160~190℃, and keep the temperature for 30~60min; in the second stage, increase the temperature from 160~190℃ to 280~350℃ at a rate of 5~10℃/min, and keep the temperature for 30~60min; in the third stage, from 280~350℃ Raise the temperature to 530-580°C at a rate of 5-10°C/min and keep it warm for 30-60 minutes; in the fourth stage, raise the temperature from 530-580°C to 730-780°C at a rate of 5-10°C/min and keep it warm for 30-60 minutes. ; In the fifth stage, the temperature is raised from 730 to 780°C to 890 to 910°C at a rate of 5 to 10°C/min, and kept warm for 55 to 70 minutes. 9. The bamboo charcoal-based methyl lactate catalyst prepared by the preparation method according to any one of claims 1 to 8. 10. Application of the bamboo charcoal-based methyl lactate catalyst prepared by the preparation method according to any one of claims 1 to 8 or the bamboo charcoal-based methyl lactate catalyst according to claim 9 in the preparation of lactic acid heterogeneous catalyst.