Catalyst for preparing lactide and synthesis method thereof

Abstract

The invention relates to a depolymerization catalyst for preparing lactide and a preparation method thereof.According to the catalyst, a honeycomb heat accumulator serves as a matrix, the surface of the honeycomb heat accumulator is coated with an aluminum oxide coating, and then the honeycomb heat accumulator is impregnated and loaded with an active component Sn; based on the mass of the honeycomb heat accumulator, the proportion of the aluminum oxide coating is 8wt%-20wt%, and the proportion of the active component is 2wt%-6wt%. The invention also provides a synthesis method of the depolymerization catalyst for preparing lactide. The catalyst provided by the invention can effectively reduce the depolymerization temperature, improve the heat transfer and mass transfer efficiency of a reaction system, reduce the racemization degree and coking carbonization probability of lactide, and effectively ensure the reaction yield and product quality.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and in particular relates to a catalyst for preparing lactide and a synthesis method thereof. Background Art

[0002] Currently, commercially available biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), and polybutylene terephthalate-adipate (PBAT). PLA is currently the most widely used and holds the greatest promise. Industrial PLA synthesis is primarily achieved through the ring-opening polymerization of lactide: in the first step, lactide is produced from lactic acid; in the second step, lactide ring-opening polymerization produces PLA. The resulting PLA can have a molecular weight ranging from 100,000 to 1 million. Lactide is crucial to the entire synthesis process, and the process barriers are relatively high. The current industrial lactide synthesis method, referred to as the "two-step process," involves the polycondensation and depolymerization of lactic acid under a catalyst, high temperature, and high vacuum. This process is prone to racemization to produce m-lactide. The presence of m-lactide can affect the optical purity of lactide, hindering the lactide ring-opening polymerization process and resulting in a lower molecular weight for the resulting PLA. Furthermore, it can disrupt the structural regularity of PLA, reducing its crystallinity and mechanical properties.

[0003] Therefore, the crude lactide currently produced by depolymerization requires purification through processes such as solvent recrystallization, water extraction, distillation, melt crystallization, and combinations thereof to reduce impurities such as m-lactide in the product. However, due to the similar physicochemical properties of L-lactide and m-lactide, and the inherent high freezing and boiling points and heat sensitivity of lactide, separation is difficult, and the overall yield is low, only around 40%-60%, and the overall economic efficiency is also low. Therefore, racemization in the lactide synthesis process is a key factor affecting lactide quality and yield, and is currently a key difficulty in lactide technology research both domestically and internationally.

[0004] The depolymerization of lactic acid oligomers to produce lactide has a high activation energy, necessitating the addition of a catalyst to reduce the activation energy of the "backbiting" process and initiate the depolymerization process. Commonly used catalysts for depolymerization include zinc and tin compounds, metal compounds, protonic acid catalysts, and rare earth catalysts. Under certain reaction conditions, the order of catalyst activity is: tin > zinc > zirconium > titanium > aluminum. The type and dosage of catalyst significantly influence the yield, chemical, and optical purity of the lactide product. Generally speaking, alkaline catalysts are more likely to induce racemization of lactide, and the greater the amount of catalyst used, the more severe the racemization.

[0005] Morteza et al. studied the effect of catalyst on the L-lactide synthesis process and found that the use of SnCl2 and sulfuric acid as catalysts resulted in the highest lactide purity and the lowest meso-lactide content. However, the catalyst content should be kept to a minimum. For example, increasing the SnCl2 content increased the lactide production rate, but excessive catalyst increased the racemization rate. Zhang et al., investigating the synthesis of lactide from lactic acid using a zinc lactate and NaHCO3 system, found that while NaHCO3, as an alkaline oxide, increased the reaction rate during the catalytic depolymerization process, it also tended to induce SN2 reactions, leading to racemization. Reducing the amount of NaHCO3 and increasing the amount of zinc lactate effectively shortened the residence time and reduced the racemization of lactide. Dong et al. investigated the effect of catalyst on the depolymerization process to lactide and found that, when tin catalysts were used, the crude lactide yield increased with increasing temperature, but the product purity decreased. The addition of tin catalysts also decreased the product purity compared to the absence of catalyst. This may be because the presence of (weak) basic catalysts can initiate and accelerate the racemization of lactide, and the increase in temperature further intensifies the racemization of lactide.

[0006] CN116063274A discloses a method for the efficient catalytic synthesis of lactide, comprising: (1) adding lactic acid oligomers and a depolymerization catalyst to a reactor, heating and raising the temperature, and adding a protonated solvent to carry out a depolymerization reaction, and collecting crude lactide; (2) purifying and refining the crude lactide obtained in step (1) to obtain a lactide product that meets polymerization grade requirements. The depolymerization catalyst is a tin compound catalyst, specifically at least one of stannous octoate, stannous chloride, stannous oxide, stannous sulfate, and stannous oxalate. This patent can inhibit the racemization of lactide during the synthesis process, while accelerating the depolymerization reaction rate and ensuring product quality and yield. However, the use of the depolymerization catalyst requires the addition of a long-chain diol solvent; and continuous operation can easily cause catalyst accumulation, further exacerbating racemization.

[0007] The reaction mechanism of lactic acid oligomer depolymerization to lactide shows that temperature is a major factor affecting product yield and purity (racemization). On the one hand, high temperatures favor the forward reaction and accelerate production efficiency; on the other hand, excessively high reaction temperatures can also exacerbate side reactions such as racemization. Therefore, the depolymerization reaction must maintain a high heat transfer efficiency to avoid the formation of local hot spots and ensure product purity. At the same time, the depolymerization process of lactic acid oligomers to lactide can be roughly viewed as a reversible reaction process, requiring high activation energy. Catalysts are generally required to accelerate the forward depolymerization reaction, inhibit the continued polymerization of oligomer molecules and the continuous growth of molecular chains, improve mass transfer efficiency, and enhance depolymerization efficiency, product quality, and yield. Summary of the Invention

[0008] To address the shortcomings of the existing technology, the present invention provides a catalyst for preparing lactide and a synthesis method thereof. The catalyst provided by the present invention can effectively lower the depolymerization temperature, improve the heat and mass transfer efficiency of the reaction system, reduce the degree of lactide racemization and the probability of coking and carbonization, and effectively ensure the reaction yield and product quality.

[0009] A first aspect of the present invention provides a depolymerization catalyst for preparing lactide. The catalyst uses a honeycomb heat accumulator as a substrate, an aluminum oxide coating is coated on the surface of the honeycomb heat accumulator, and then an active component Sn is impregnated and loaded. Based on the mass of the honeycomb heat accumulator, the aluminum oxide coating accounts for 8wt%-20wt%, and the active component SnO accounts for 2wt%-6wt%.

[0010] In the catalyst of the present invention, the honeycomb heat accumulator is any one or more of cordierite honeycomb ceramics, mullite honeycomb ceramics, etc. The pore size of the honeycomb heat accumulator is 1.0-4.0 mm.

[0011] In the catalyst of the present invention, further, a composite coating containing both aluminum oxide and graphene oxide is coated on the surface of the honeycomb heat storage body, and the proportion of the graphene oxide is 0.1wt%-1.0wt%.

[0012] A second aspect of the present invention provides a method for synthesizing a depolymerization catalyst for preparing lactide, comprising the following steps:

[0013] (1) Dipping the honeycomb heat storage body into alumina sol, blowing out the residual liquid in the pores with compressed air, and drying and calcining to obtain a carrier coated with alumina;

[0014] (2) Immerse the carrier in an active component solution containing tin salt, take it out and blow away the residual liquid with compressed air, and then obtain the catalyst after drying and calcining.

[0015] In the synthesis method of the present invention, the honeycomb thermal accumulator in step (1) is at least one of cordierite honeycomb ceramics, mullite honeycomb ceramics, and the like. The pore size of the honeycomb thermal accumulator is 1.0-4.0 mm.

[0016] In the synthesis method of the present invention, a non-limiting method for preparing the alumina sol described in step (1) is as follows: pseudo-boehmite is added to deionized water, and concentrated nitric acid is added dropwise while stirring; after stirring for a certain period of time, the mixture is heated to 50-90° C., nitric acid is added dropwise until complete peptization is achieved, the pH of the solution is controlled to 2-5, and the mixture is aged for 12-48 hours to obtain a transparent alumina sol. The mass ratio of the pseudo-boehmite to deionized water is 1:2-1:10, and the mass ratio of concentrated nitric acid to pseudo-boehmite is 1:5-3:5.

[0017] In the synthesis method of the present invention, the time for immersing the honeycomb heat storage body in the alumina sol in step (1) is 3-10 minutes.

[0018] In the synthesis method of the present invention, the drying temperature in step (1) is 80-120° C. and the drying time is 5-8 hours; the roasting temperature is 400-600° C. and the roasting time is 2-10 hours.

[0019] In the synthesis method of the present invention, in the active component solution containing tin salt described in step (2), the tin salt can be at least one of stannous octoate, stannous chloride, stannous sulfate, stannous oxalate, etc., and the concentration of the tin salt in the solution is 10-50g / L.

[0020] In the synthesis method of the present invention, the immersion time in step (2) is 5-20 minutes.

[0021] In the synthesis method of the present invention, the drying temperature in step (2) is 80-120° C. and the drying time is 5-8 hours; the calcination process is carried out under an inert atmosphere at a temperature of 400-600° C. and the calcination time is 2-10 hours. The inert atmosphere can be achieved by introducing nitrogen, an inert gas, etc.

[0022] In the synthesis method of the present invention, graphene oxide is further added to the alumina sol to produce a graphene oxide-alumina sol; the mass ratio of the added graphene oxide to the alumina is 1:8-1:100. A preferred addition method is: after dropwise addition of nitric acid until complete peptization, a graphene oxide ethanol suspension is added, followed by stirring for 2-5 hours and aging for 10-48 hours to produce the graphene oxide-alumina sol.

[0023] In the synthesis method of the present invention, the graphene oxide ethanol suspension is prepared by adding graphene oxide to ethanol and ultrasonically dispersing the mixture for 0.5-2.0 hours; the ultrasonic power is 250-1000W and the ultrasonic temperature is 40-60°C. The mass ratio of graphene oxide to ethanol is 1:100-1:500.

[0024] In the synthesis method of the present invention, the graphene oxide is prepared by using graphite as a raw material by adopting the Hummers method or purchased commercially.

[0025] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0026] (1) The present invention loads the tin oxide active component on a honeycomb heat storage body to prepare a depolymerization catalyst for preparing lactide. On the one hand, it can increase the contact area with the reactant system, improve the catalytic reaction activity, and thus reduce the depolymerization reaction temperature; on the other hand, the honeycomb heat storage body can also play a good heat transfer effect, homogenize the reaction temperature, avoid local overheating causing racemization and coking and carbonization of lactic acid oligomers, effectively improve the reaction yield, ensure product quality, and help maintain long-term stable operation of the reaction.

[0027] (2) The present invention co-coats graphene oxide and aluminum oxide on a ceramic heat storage body and then loads active components. On the one hand, the oxygen-containing functional groups on the graphene surface give it high surface activity, and the active components have high dispersion, which can better adsorb lactic acid oligomers and improve the catalytic effect; on the other hand, graphene oxide has excellent heat transfer performance, which helps to homogenize the reaction temperature, avoid racemization, and improve the catalytic effect. DETAILED DESCRIPTION

[0028] The following examples further illustrate the technical solution of the present invention and its effects. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following examples.

[0029] The present invention uses a Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. Conventional calibration methods are used, with polystyrene (PS) as the internal standard. A T3000 column, 300 mm × 8.0 mm in size, is used. The column temperature is 40°C, the flow rate is 1.0 mL / min, the sample concentration is 2-5 mg / mL, and the single injection volume is 500 μL.

[0030] The present invention uses an Agilent high-performance liquid chromatograph to analyze the chemical purity of lactide, using a UV detector, phosphoric acid and acetonitrile as the mobile phase, and a ZORBAX SB-Aq column with a length of 250 mm, an inner diameter of 4.6 mm, and a filler particle size of 5 μm. The detection wavelength is 200 nm, the column temperature is 40°C, the flow rate is 1 mL / min, and the injection volume is 5 μL.

[0031] The present invention adopts Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects CYCLOSIL-B model chromatographic column, vaporization chamber temperature 250°C, detector temperature 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature 100°C, hold 5min, heat to 140°C at a rate of 4°C / min, hold 7min, heat to 200°C at a rate of 8°C / min, hold 20min, carrier gas N2 flow rate 1.4mL / min, hydrogen flow rate 30mL / min, air flow rate 400mL / min, injection volume 0.5μL.

[0032] The yield Y of the entire reaction process is the ratio of the actual amount of L-lactide produced to the theoretical amount produced, and the calculation formula is as follows:

[0033]

[0034] Where m0 is the production rate of lactide produced from the side line, y0 is the purity of L-lactide in the lactide produced from the side line, and M is the mass of lactide that can theoretically be converted from a certain amount of lactic acid oligomers, that is, the feed rate of fresh lactic acid oligomers.

[0035] Example 1

[0036] (1) Preparation of a carrier coated with an alumina coating: Pseudoboehmite and deionized water were mixed in a mass ratio of 1:5, and concentrated nitric acid was gradually added dropwise while stirring. After uniform stirring, the mixture was heated to 70°C, and concentrated nitric acid was added dropwise until complete peptization was achieved. The pH value of the solution was adjusted to 3.1, and the amount of concentrated nitric acid added was 0.3 times the mass of the pseudoboehmite. The mixture was aged for 24 hours to obtain a transparent aluminum sol. A cordierite honeycomb ceramic thermal storage body (pore size 2.0-3.0 mm) was then immersed in the transparent aluminum sol for 10 minutes and then removed. The residual liquid in the pores was blown away with compressed air. The mixture was dried in a desiccator at 100°C for 6 hours and then transferred to a muffle furnace for calcination at 450°C for 6 hours to obtain a carrier coated with an alumina coating.

[0037] (2) Loading active components: Prepare a stannous octoate solution with a concentration of 25 g / L, then immerse the carrier coated with alumina into the stannous octoate solution for 15 min. After taking it out, use compressed air to blow away the residual liquid in the pores of the heat storage body. After drying it in a dryer at 100°C for 6 h, transfer it to a muffle furnace and calcine it at 450°C for 6 h under a N2 atmosphere to obtain a catalyst.

[0038] According to testing, based on the mass of the honeycomb heat storage body, the proportion of the aluminum oxide coating is 15wt%, and the proportion of the active component SnO is 3.5wt%.

[0039] Example 2

[0040] (1) Preparation of a carrier coated with an alumina coating: Pseudoboehmite and deionized water were mixed in a mass ratio of 1:2, and concentrated nitric acid was gradually added dropwise while stirring. After uniform stirring, the mixture was heated to 50°C, and concentrated nitric acid was added dropwise until complete peptization was achieved. The pH value of the solution was adjusted to 2.5, and the amount of concentrated nitric acid added was 0.2 times the mass of the pseudoboehmite. The mixture was aged for 24 hours to obtain a transparent aluminum sol. A cordierite honeycomb ceramic thermal storage body (pore size 3.0-4.0 mm) was then immersed in the transparent aluminum sol for 5 minutes and then removed. The residual liquid in the pores was blown away with compressed air. After drying in a desiccator at 80°C for 8 hours, the mixture was transferred to a muffle furnace and calcined at 600°C for 3 hours to obtain a carrier coated with an alumina coating.

[0041] (2) Loading active components: Prepare a stannous chloride solution with a concentration of 15 g / L, then immerse the carrier coated with aluminum oxide into the stannous octoate solution for 10 min. After taking it out, use compressed air to blow away the residual liquid in the pores of the heat storage body. After drying it in a dryer at 120°C for 5 h, transfer it to a muffle furnace and calcine it at 400°C for 10 h under a N2 atmosphere to obtain a catalyst.

[0042] After testing, based on the mass of the honeycomb heat storage body, the proportion of the aluminum oxide coating is 18.7wt%, and the proportion of the active component SnO is 2.0wt%.

[0043] Example 3

[0044] (1) Preparation of a carrier coated with aluminum oxide: Pseudoboehmite and deionized water were mixed in a mass ratio of 1:10, and concentrated nitric acid was gradually added dropwise while stirring. After uniform stirring, the mixture was heated to 90°C, and concentrated nitric acid was added dropwise until complete peptization was achieved. The pH value of the solution was adjusted to 4.7, and the amount of concentrated nitric acid added was 0.6 times the mass of the pseudoboehmite. The mixture was aged for 24 hours to obtain a transparent aluminum sol. A cordierite honeycomb ceramic thermal storage body (pore size 1.0-2.0 mm) was then immersed in the transparent aluminum sol for 8 minutes and removed. The residual liquid in the pores was blown away with compressed air. The mixture was dried in a desiccator at 120°C for 5 hours and then transferred to a muffle furnace for calcination at 500°C for 5 hours to obtain a carrier coated with Al2O3.

[0045] (2) Loading active components: Prepare a stannous sulfate solution with a concentration of 50 g / L, and then immerse the carrier coated with Al2O3 into the stannous octoate solution for 10 min. After taking it out, use compressed air to blow away the residual liquid in the pores of the heat storage body. After drying it in a dryer at 80°C for 8 h, transfer it to a muffle furnace and calcine it at 600°C for 4 h under N2 atmosphere to obtain the catalyst.

[0046] According to the test, based on the mass of the honeycomb heat storage body, the proportion of the aluminum oxide coating is 8.2wt%, and the proportion of the active component SnO is 5.8wt%.

[0047] Example 4

[0048] The catalyst is prepared in the same manner as in Example 1, except that a mullite honeycomb ceramic heat storage body is used instead of the cordierite honeycomb ceramic.

[0049] According to the test, based on the mass of the honeycomb heat storage body, the proportion of the aluminum oxide coating is 15.6wt%, and the proportion of the active component SnO is 3.2wt%.

[0050] Example 5

[0051] The same as Example 1, except that: graphene oxide-alumina sol is used instead of alumina sol to finally obtain a catalyst. The preparation process of graphene oxide-alumina sol is as follows: commercially purchased graphene oxide is taken, added to ethanol according to the mass ratio of graphene oxide and ethanol being 1:100, ultrasonic dispersion, ultrasonic power 500W, ultrasonic temperature 50°C, ultrasonic time 1.0h, to obtain a graphene oxide ethanol suspension, after dripping nitric acid to complete peptization, the graphene oxide ethanol suspension is added to the alumina sol after complete peptization, wherein the addition amount of graphene oxide is 0.04 times the mass of alumina. After stirring for another 3 hours, ageing for 24 hours to obtain graphene oxide-alumina sol.

[0052] After testing, based on the mass of the honeycomb thermal storage body, the proportion of the aluminum oxide coating is 14.8wt%, the proportion of the graphene oxide is 0.59wt%, and the proportion of the active component SnO is 3.8wt%.

[0053] Example 6

[0054] With Example 2, except that: graphene oxide-alumina sol is used instead of alumina sol, and catalyst is finally obtained. The preparation process of graphene oxide-alumina sol is as follows: commercially purchased graphene oxide is taken, added to ethanol according to the mass ratio of graphene oxide and ethanol being 1: 250, ultrasonic dispersion, ultrasonic power 250W, ultrasonic temperature 40 DEG C, ultrasonic time 2.0h, to obtain graphene oxide ethanol suspension, after dripping nitric acid to complete peptization, graphene oxide ethanol suspension is added to the alumina sol after complete peptization, wherein the addition amount of graphene oxide is 0.016 times of the mass of alumina. After stirring for another 3 hours, ageing for 24 hours to obtain graphene oxide-alumina sol.

[0055] After testing, based on the mass of the honeycomb thermal storage body, the proportion of the aluminum oxide coating is 18.8wt%, the proportion of the graphene oxide is 0.3wt%, and the proportion of the active component SnO is 2.3wt%.

[0056] Example 7

[0057] With Example 3, except that: graphene oxide-alumina sol is used instead of alumina sol to finally obtain a catalyst. The preparation process of graphene oxide-alumina sol is as follows: commercially purchased graphene oxide is taken, added to ethanol according to the mass ratio of graphene oxide and ethanol being 1:500, ultrasonic dispersion, ultrasonic power 100W, ultrasonic temperature 60 ° C, ultrasonic time 0.5h, to obtain graphene oxide ethanol suspension, after dripping nitric acid to complete peptization, the graphene oxide ethanol suspension is added to the alumina sol after complete peptization, wherein the addition amount of graphene oxide is 0.09 times of the mass of alumina. After stirring for another 3 hours, ageing for 24 hours to obtain graphene oxide-alumina sol.

[0058] After testing, based on the mass of the honeycomb thermal storage body, the proportion of the aluminum oxide coating is 8.7wt%, the proportion of the graphene oxide is 0.78wt%, and the proportion of the active component SnO is 6.0wt%.

[0059] Comparative Example 1

[0060] The same as Example 1, except that the honeycomb thermal storage body is replaced with alumina of similar pore size, and no alumina coating is prepared. The active component is directly impregnated to obtain the catalyst. Based on the mass of the alumina, the proportion of the active component SnO is 3.8 wt%.

[0061] Comparative Example 2

[0062] The same as Example 1, except that the honeycomb thermal storage element was replaced with a molecular sieve of similar pore size, and no alumina coating was prepared. The active component was directly impregnated to obtain the catalyst. The active component SnO accounted for 3.0 wt% based on the mass of the molecular sieve.

[0063] Comparative Example 3

[0064] The catalyst was prepared in the same manner as in Example 1, except that graphene oxide was used instead of aluminum oxide. Based on the mass of the honeycomb thermal storage body, the graphene oxide accounted for 8.3% and the active component SnO accounted for 2.8 wt%.

[0065] Comparative Example 4

[0066] The catalyst was prepared in the same manner as in Example 1, except that the active component was directly loaded onto the honeycomb thermal storage body without preparing an alumina coating. The active component, SnO, accounted for 1.7 wt % based on the mass of the honeycomb thermal storage body.

[0067] Test Case

[0068] Application of the catalysts prepared in the examples and comparative examples of the present invention in the synthesis of lactide.

[0069] The depolymerization of L-lactic acid oligomers with a weight-average molecular weight of 1600 was investigated in a tank reactor. The catalyst was added at a concentration of 10% of the total weight of the lactic acid oligomers. The reaction temperature was 190-210°C and the vacuum was 600 Pa. The experimental results are shown in Table 1.

[0070] Table 1

[0071]

[0072] The test results of the Examples and Comparative Examples show that, because the Comparative Examples do not employ the complete technical solution of the present invention, heat transfer is uneven, resulting in higher temperatures near the reactor walls and lower temperatures at the center. This not only affects catalyst efficiency but also easily leads to localized overheating, resulting in severe racemization and substrate coking, thus affecting yield and product quality. However, the technical solution of the present invention improves yield while suppressing lactide racemization.

Claims

1. A depolymerization catalyst for preparing lactide, characterized in that: The catalyst uses a honeycomb heat accumulator as a matrix, an aluminum oxide coating is coated on the surface of the honeycomb heat accumulator, and then the active component Sn is impregnated and loaded. Based on the mass of the honeycomb heat accumulator, the aluminum oxide coating accounts for 8wt%-20wt%, and the active component SnO accounts for 2wt%-6wt%.

2. The catalyst according to claim 1, characterized in that: The honeycomb heat storage body is any one or more of cordierite honeycomb ceramics and mullite honeycomb ceramics.

3. The catalyst according to claim 1 or 2, characterized in that: The pore diameter of the honeycomb heat storage body is 1.0-4.0 mm.

4. The catalyst according to claim 1 or 2, characterized in that: A composite coating containing both aluminum oxide and graphene oxide is coated on the surface of the honeycomb heat storage body, wherein the proportion of the graphene oxide is 0.1 wt % to 1.0 wt %.

5. A method for synthesizing the depolymerization catalyst for preparing lactide according to any one of claims 1 to 3, characterized in that The steps include: (1) Immerse the honeycomb heat storage body in alumina sol, remove it and blow away the residual liquid in the pores with compressed air, and then dry and calcine to obtain a carrier coated with alumina; (2) Immerse the carrier in a solution of active components containing tin salts, take it out and blow away the residual liquid with compressed air. After drying and calcining, the catalyst is obtained.

6. The synthesis method according to claim 5, characterized in that: The honeycomb heat storage body in step (1) is at least one of cordierite honeycomb ceramics and mullite honeycomb ceramics.

7. The synthesis method according to claim 5 or 6, characterized in that: The pore size of the honeycomb heat storage body in step (1) is 1.0-4.0 mm.

8. The synthesis method according to claim 5, wherein: The preparation method of the alumina sol in step (1) is as follows: adding pseudo-boehmite to deionized water, and adding concentrated nitric acid dropwise while stirring; after stirring for a certain period of time, heating to 50-90° C., adding nitric acid dropwise until complete peptization, controlling the pH value of the solution to 2-5, aging for 12-48 hours, and obtaining a transparent aluminum sol; the mass ratio of the pseudo-boehmite to deionized water is 1:2-1:10, and the mass ratio of concentrated nitric acid to pseudo-boehmite is 1:5-3:

5.

9. The synthesis method according to claim 5 or 6, characterized in that: The time for immersing the honeycomb heat storage body in the alumina sol in step (1) is 3-10 minutes.

10. The synthesis method according to claim 5, characterized in that: The drying temperature in step (1) is 80-120° C., and the drying time is 5-8 hours; the roasting temperature is 400-600° C., and the roasting time is 2-10 hours.

11. The method according to claim 5, wherein: In the active component solution containing tin salt described in step (2), the tin salt is at least one of stannous octoate, stannous chloride, stannous sulfate, and stannous oxalate, and the concentration of the tin salt in the solution is 10-50 g / L.

12. The method according to claim 5, wherein: The impregnation time in step (2) is 5-20 minutes; the drying temperature is 80-120°C, and the drying time is 5-8 hours; the roasting process is carried out under an inert atmosphere, the roasting temperature is 400-600°C, and the roasting time is 2-10 hours.

13. A method for synthesizing the depolymerization catalyst for preparing lactide according to claim 4, characterized in that: Based on the synthesis method according to any one of claims 5 to 12, the method further comprises the following steps: adding graphene oxide to the alumina sol to prepare graphene oxide-alumina sol; the mass ratio of the added amount of graphene oxide to alumina is 1:8-1:

100.

14. The method according to claim 13, wherein: The preferred method of adding graphene oxide is: after adding nitric acid dropwise until it is completely peptized, adding graphene oxide ethanol suspension, stirring for 2-5 hours, and aging for 10-48 hours to obtain graphene oxide-alumina sol.

15. The method according to claim 14, characterized in that: The graphene oxide ethanol suspension is prepared by adding graphene oxide to ethanol and ultrasonically dispersing the mixture for 0.5-2.0 hours, with an ultrasonic power of 250-1000W and an ultrasonic temperature of 40-60°C, wherein the mass ratio of graphene oxide to ethanol is 1:100-1:500.

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