Method for preparing lactide through dehydration and cyclization of lactic acid

Abstract

The invention relates to a method for preparing lactide by dehydrating and cyclizing lactic acid, which comprises the following steps: (1) heating a lactic acid solution to react, and dehydrating to produce a lactic acid dimer; (2) carrying out low-temperature negative-pressure dehydration treatment on a product obtained in the step (1); and (3) carrying out contact reaction on the product in the step (2) and a catalyst to produce lactide, and taking measures to remove water generated in the reaction in the reaction process. Aiming at the problems of a two-step method and a one-step method in the lactide production process in the prior art, the invention provides a brand-new process method for preparing lactide by dehydrating and cyclizing lactic acid through two steps, most lactic acid is pre-polymerized and converted into a dimer by controlling reaction conditions and reaction balance, so that the generation of a lactic acid polymer can be reduced, and the yield of lactide is improved; the effective conversion rate of lactic acid is improved; the lactic acid dimer is dehydrated and then cyclized to produce the lactide, so that the dehydration pressure of a reaction system can be greatly reduced, the hydrolysis reaction of the lactide is reduced, and the yield of the lactide is increased.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a method for preparing lactide from lactic acid, and more specifically to a method for preparing lactide by dehydration and cyclization of lactic acid. Background Technology

[0002] Traditional single-use plastic products are physically and chemically stable and have a long natural degradation time. Currently, the global annual consumption of single-use plastic products reaches 120 million tons, with only 10% being recycled. The rest is incinerated or discarded into the soil, air, and oceans. Both incineration and disposal cause irreversible damage to ecosystems, seriously endangering the health and safety of land, water bodies, animals, and humans. Nearly 90 countries and regions worldwide have introduced policies or regulations to control or ban single-use non-degradable plastic products.

[0003] Polylactic acid (PLA), a biodegradable plastic, is currently the most widely used and has the most promising application prospects. It not only possesses the basic properties of general polymer materials, but also excels in processing performance, physical and mechanical properties, and biodegradability. It can be widely used in the packaging, textile, agricultural, and consumer goods markets, and is considered the most likely biodegradable material to replace petroleum-based polyesters. Industrial-scale PLA synthesis mainly involves the ring-opening polymerization of lactide, which is the final step in the PLA process.

[0004] Currently, lactide is prepared by polycondensation and depolymerization of lactic acid in a high-temperature, high-vacuum system using a catalyst. However, this process easily leads to racemization of lactide, reducing the selectivity of L-lactide. Furthermore, the accumulation of liquid-phase catalyst during the reaction, which cannot be separated from the system, further exacerbates the racemization of lactide. This causes the carboxylic acid anion at the end of the lactic acid oligomer to attack the chiral carbon atom of the unit adjacent to the lactic acid unit, resulting in bond breakage between the methylene carbon and the ester oxygen bond (a "reverse bite" process), leading to configuration inversion and the formation of meso-lactide (m-lactide). The presence of m-lactide affects the optical purity of lactide, thus impacting the ring-opening polymerization process. This results in lower molecular weight, reduced crystallinity, and decreased mechanical properties in the produced PLA, representing a key focus and challenge in current lactide technology research both domestically and internationally. Currently, many patents focus on the technology of directly synthesizing lactide from lactic acid in one step. However, due to the heat-sensitive properties of lactic acid, and the fact that the two steps of the "one-pot" method—dehydrating lactic acid to obtain lactic acid dimer and then dehydrating and cyclizing it to generate lactide—affect each other and have different optimal reaction conditions, the results are not ideal.

[0005] CN200910258665.1 discloses a catalyst for the direct preparation of lactide and a method for the direct preparation of lactide using the catalyst. The lactide is a cyclic ester of a monomer used in polylactide. The method includes: in an inert environment, in the presence of a titanium-based catalyst or a mixture of catalysts containing the titanium-based catalyst, a transesterification reaction between two molecules of a lactate ester or a mixture containing the lactate ester and a small amount of lactic acid and lactic acid oligomers to prepare lactide, while simultaneously removing alcohols generated as byproducts. In this method, the formation of lactic acid polymers cannot be effectively suppressed by organic solvents alone, thus significantly reducing the selectivity of lactide.

[0006] CN201810612564.9 discloses a method for preparing lactide, comprising: heating lactic acid in the presence of H-beta molecular sieve under reduced pressure to obtain lactide; the reaction pressure is 50-1500 Pa. This invention employs a vacuum and heating method to achieve the H-beta molecular sieve-catalyzed lactic acid reaction, directly synthesizing lactic acid into lactide, obtaining lactide with high optical purity (>96%). This overcomes the shortcomings of existing lactide production methods, which require oligomerization and high-temperature pyrolysis, resulting in long process flows and high reaction temperatures. The entire production process does not use organic solvents, and only wastewater is generated, causing minimal environmental pollution. The reaction temperature is low, and repeated catalysis is achieved through the hydrolysis of lactic acid oligomers, allowing the H-beta molecular sieve to be recycled. This method uses 100-105% concentration lactic acid without organic solvents. High-concentration lactic acid has poor fluidity, low mass transfer efficiency, and easily forms polymers that clog catalyst pores.

[0007] CN201911071750.7 discloses a method for the shape-selective synthesis of lactide using a zeolite-supported catalyst. It uses lactic acid or lactate esters as substrates and a zeolite-supported Lewis acid as a catalyst for the efficient synthesis of lactide. Compared with existing technologies, this invention significantly shortens the reaction time, increases the yield, and reduces the production cost of lactide by employing a defined method. The previous method used toluene and xylene as organic solvents, which are immiscible with lactide and other products, resulting in poor mass transfer, low reactivity, and difficulty in subsequent product extraction.

[0008] CN114853719A discloses a method and apparatus for enhancing the one-step production of lactic acid from lactide, providing a means and apparatus for recovering and utilizing azeotropic agents within the apparatus and improving the single-pass conversion rate of the reaction. Using L-lactic acid or D-lactic acid as raw material, a lactic acid dehydration reaction is carried out in a boiling organic solvent under the action of a catalyst. The vapor generated during the reaction is condensed and subjected to phase separation. During the phase separation process, one or more combinations of salt chemicals and physical dehydrating agents are added, and the distilled organic solvent is continuously refluxed back to the reactor, ultimately yielding an organic solvent containing L-lactide or D-lactide products. During the reaction, the organic solvent continuously removes water generated during the reaction through azeotropy. The method of adding salts and physical dehydrating agents in this invention can remove trace amounts of water from the refluxed organic solvent, while simultaneously improving the single-pass conversion rate and rate of the reaction, and reducing the production cost of the one-step lactic acid production of lactide. This method reduces the influence of water on the reaction system by coupling salting-out technology with azeotropic distillation technology, but it cannot strictly control the synthesis route of lactide. After a long time, the yield of lactide decreases and the amount of lactic acid polymers increases. Summary of the Invention

[0009] Through the inventors' research on the one-step production process of lactide, the following problems were found: Firstly, because lactide is a heat-sensitive substance, water is generated during both lactic acid dehydration and cyclization. If the water generated during lactic acid dehydration is not removed from the system in time, it will further lead to lactide hydrolysis and ring-opening at higher temperatures, reducing the yield of lactide and increasing the content of polymers. Therefore, rapidly removing water from the system is a key factor in improving the yield of lactide. Secondly, only lactic acid dimers can cyclize to generate lactide monomers under the confinement effect of a solid catalyst. The content of lactic acid dimers in the system determines the final yield of lactide. Regarding the first aspect, the existing one-step method for producing lactide typically takes place in a single reactor. Water introduced by the lactic acid solution, water generated during dehydration, and water produced during cyclization are all present in the reaction system, placing significant pressure on timely dehydration. Insufficient dehydration will still negatively impact the lactide yield to some extent. Regarding the second aspect, since lactic acid dehydration and cyclization occur simultaneously in the reactor, it is difficult to control the formation of lactic acid dimers, resulting in a low lactide yield.

[0010] Therefore, to address the shortcomings of existing technologies, this invention provides a method for preparing lactide through lactic acid dehydration and cyclization. By employing a stepwise reaction method and strictly controlling reaction conditions, compared to the traditional two-step lactic acid polycondensation-depolymerization process for preparing lactide, this method solves the racemization problem and easily addresses the issue of catalyst recovery and reuse. This process requires less sophisticated equipment and is less costly. Compared to the one-step direct synthesis of lactide from lactic acid in a single reactor, this method controls the efficient formation of lactic acid dimers and dehydrates them before cyclization, reducing the dehydration pressure on the system and improving lactide yield and catalyst lifetime.

[0011] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0012] A method for preparing lactide by dehydration and cyclization of lactic acid, comprising:

[0013] (1) Heat the lactic acid solution to react and dehydrate it to produce lactic acid dimer;

[0014] (2) The product from step (1) is subjected to low-temperature negative pressure dehydration treatment;

[0015] (3) The product of step (2) is reacted with a catalyst to produce lactide. During the reaction, measures are taken to remove the water produced in the reaction.

[0016] Furthermore, the concentration of the lactic acid solution is 20wt%-80wt%, preferably 50wt%-80wt%. The heating reaction temperature is 80-120℃, and the reaction time is 1-3h.

[0017] Furthermore, the reaction in step (1) to produce lactic acid dimer by dehydrating lactic acid is carried out in a reactor equipped with a drainage device to remove some of the water from the system during the reaction. The drainage device is used to control the water content of the substrate in the reaction system to be maintained at 20-50%.

[0018] Furthermore, the reaction of lactic acid dehydration to produce lactic acid dimer in step (1) is carried out in a reactor connected to a vacuum drainage system, and the pressure of the reactor is controlled at 60-85 kPa.

[0019] Furthermore, the low-temperature negative pressure dehydration treatment removes water from the lactic acid dimer by controlling the temperature at 40-60℃ and the vacuum degree at 4-20Kpa.

[0020] Furthermore, the low-temperature negative pressure dehydration treatment dehydrates the substrate to a moisture content of less than 8%.

[0021] Furthermore, the reaction in step (3) is preferably carried out in the form of a fixed bed or catalytic distillation column. The catalyst used in this reaction is well known to those skilled in the art, and is more preferably a solid catalyst, which is beneficial for continuous production. More specifically, the catalyst is preferably a zeolite catalyst, preferably at least one of HY, H-USY, SAPO-34, titanium silicate molecular sieve and H-Beta.

[0022] Furthermore, the measures taken to remove water generated in step (3) include, but are not limited to, solvent removal and air stripping methods. The solvent is a reaction-inert solvent selected from at least one of benzene, toluene, xylene, methyl isobutyl ketone, cyclohexanone, and cyclohexane, preferably toluene.

[0023] Furthermore, the reaction process in step (3) is preferably implemented by a catalytic distillation column: the catalyst is packed into the middle reaction section of the catalytic distillation column, the product of step (2) is introduced into the catalytic distillation column from the top, the organic solvent or reaction inert gas is introduced from the bottom of the catalytic distillation column, and after passing through the middle reaction section, the water removed by the reaction is carried out from the top of the column, and the reaction product containing lactide is collected from the bottom of the column.

[0024] Furthermore, the catalyst in the intermediate reaction section of the catalytic distillation column is also filled with inert packing material at both the top and bottom.

[0025] Furthermore, in step (3), the volume ratio of the reaction raw materials to the organic solvent entering the catalytic distillation column per unit time is 1:3-10.

[0026] Furthermore, the operating pressure of the catalytic distillation column is 0.05-0.3 MPa.

[0027] Furthermore, the temperature of the intermediate reaction section of the catalytic distillation column is 100℃-140℃.

[0028] Furthermore, the ratio of the mass flow rate of the reactant entering the catalytic distillation column to the mass of the catalyst is 1-10 h⁻¹. -1 Preferably 1-8h -1 .

[0029] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0030] (1) In view of the problems of the two-step and one-step methods of lactide production in the prior art, the present invention proposes a new two-step process for the dehydration and cyclization of lactic acid to produce lactide. First, by controlling the reaction conditions and reaction balance, most of the lactic acid prepolymer is converted into dimer, which can reduce the generation of lactic acid polymer (n≥3) and improve the effective conversion rate of lactic acid. After the lactic acid dimer is dehydrated, lactide is produced by cyclization, which can greatly reduce the dehydration pressure of the reaction system, reduce the hydrolysis reaction of lactide, and increase the yield of lactide.

[0031] (2) In the preferred embodiment, the cyclization process can be achieved by using a catalytic distillation column, which is conducive to the continuous production of the entire reaction. Moreover, the amount of lactic acid polymer is small, which is not easy to block the catalyst channels, greatly extending the service life of the catalyst and further improving the production efficiency. In addition, the solid form of the catalyst reduces the corrosion of the equipment and lowers the equipment requirements.

[0032] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0033] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0034] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0035] Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples can be purchased from biochemical reagent stores.

[0036] This invention utilizes an Agilent high-performance liquid chromatograph (with UV detector) to analyze the chemical purity of lactide and the analysis of lactic acid dimer and polymer. Phosphoric acid and acetonitrile are used as the mobile phase. The chromatographic column is a ZORBAX SB-Aq, 250 mm long, 4.6 mm inner diameter, and packed with 5 μm particle size. The detection wavelength is 200 nm, column temperature is 40 °C, flow rate is 1 ml / min, and injection volume is 5 μL.

[0037] Lactic acid conversion rate % = (1 - weight of unreacted lactic acid in the final product / initial weight of lactic acid feed) × 100%

[0038] Lactide yield % = Weight of lactide in final product / (Initial lactic acid feed weight × 144 / 180) × 100%

[0039] Example 1

[0040] (1) Add a 60% L-lactic acid solution to the reactor and react at 100°C for 2 hours. The reactor is equipped with a vacuum drainage device and the vacuum degree is controlled at 70 kPa to drain excess water.

[0041] (2) The product after reaction (1) was subjected to rotary evaporation at 50℃ and 10kPa to remove water, so that the water content was reduced to below 8%; the content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1;

[0042] (3) The reaction product from (2) was added from the top of the catalytic distillation column at a rate of 1.2 kg / h (based on the amount of lactic acid / lactic acid dimer), and toluene solution was introduced from the bottom of the reactor at a rate of 7 kg / h. The operating pressure of the catalytic distillation column was atmospheric pressure. The intermediate reaction section of the column was filled with a uniformly mixed stainless steel wire mesh bag and Φ3×3 mm stainless steel θ mesh rings. The stainless steel wire mesh bag contained 0.5 kg of H-Beta catalyst. The temperature of the intermediate reaction section was controlled at 120 °C, and crude lactide was obtained in the bottom of the column. The content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1.

[0043] Example 2

[0044] (1) Add a 50% L-lactic acid solution to the reactor and react at 80°C for 1 hour. The reactor is equipped with a vacuum drainage device and the vacuum degree is controlled at 60 kPa.

[0045] (2) The product after reaction (1) was subjected to rotary evaporation at 40℃ and 4kPa to remove water, so that the water content was reduced to below 8%; the content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1;

[0046] (3) The reaction product from (2) was added from the top of the catalytic distillation column at a rate of 1.2 kg / h (based on the amount of lactic acid / lactic acid dimer), and toluene solution was introduced from the bottom of the reactor at a rate of 3.6 kg / h. The operating pressure of the catalytic distillation column was 0.05 MPa. The intermediate reaction section of the column was filled with a uniformly mixed stainless steel wire mesh bag and Φ3×3 mm stainless steel θ mesh rings. The stainless steel wire mesh bag contained a total of 1.2 kg of H-Beta catalyst. The temperature of the intermediate reaction section was controlled at 100 °C. Crude lactide was obtained in the bottom of the column. The content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1.

[0047] Example 3

[0048] (1) Add an 80% L-lactic acid solution to the reactor and react at 120°C for 3 hours. The reactor is equipped with a vacuum drainage device and the vacuum degree is controlled at 85 kPa.

[0049] (2) The product after reaction (1) was subjected to rotary evaporation at 60℃ and 20kPa to remove water, so that the water content was reduced to below 8%; the content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1;

[0050] (3) The reaction product from (2) was added from the top of the catalytic distillation column at a rate of 1.2 kg / h (based on the amount of lactic acid / lactic acid dimer), and toluene solution was introduced from the bottom of the reactor at a rate of 12 kg / h. The operating pressure of the catalytic distillation column was 0.3 MPa. The intermediate reaction section of the column was filled with a uniformly mixed stainless steel wire mesh bag and Φ3×3 mm stainless steel θ mesh rings. The stainless steel wire mesh bag contained a total of 0.17 kg of H-Beta catalyst. The temperature of the intermediate reaction section was controlled at 140 °C. Crude lactide was obtained in the bottom of the column. The content of each component in the product was analyzed by high performance liquid chromatography, as shown in Table 1.

[0051] Comparative Example 1

[0052] Skipping steps (1) and (2), L-lactic acid with a mass concentration of 60% is directly passed into the catalytic distillation column, which is equivalent to the conventional one-step preparation of lactide from lactic acid in a single reactor in the prior art. Specifically:

[0053] A 60% L-lactic acid solution was added to the top of the catalytic distillation column at a rate of 1.2 kg / h (based on lactic acid). Toluene solution was introduced into the bottom of the reactor at a rate of 7 kg / h. The reactor setup and reaction conditions were the same as in Example 1. Crude lactide was obtained in the bottom of the column. The content of each component in the product was analyzed by high-performance liquid chromatography (HPLC), as shown in Table 1.

[0054] Comparative Example 2

[0055] The process is the same as in Example 1, except that in step (1), the reactor does not have a vacuum drainage system, the water content of the product is not controlled, and the reaction is carried out under normal pressure.

[0056] Comparative Example 3

[0057] The process is the same as in Example 1, except that the low-temperature dehydration process in step (2) is omitted. The product with a water content of 25% obtained in step (1) is fed into the same catalytic distillation column as in step (3) at a rate of 1.2 kg / h based on the amount of lactic acid / lactic acid dimer.

[0058] Comparative Example 4

[0059] The process is the same as in Example 1, except that in step (3), lactide is synthesized using a batch reactor.

[0060] Table 1

[0061]

[0062] All percentages in Table 1 are by mass and do not include the mass of water. In particular, the products from step (2) of Examples 1-3 contain virtually no lactic acid polymers (n≥3).

Claims

1. A method for preparing lactide by dehydration and cyclization of lactic acid, comprising: (1) The lactic acid solution is heated to react and dehydrate it to produce lactic acid dimer; (2) The product from step (1) is subjected to low-temperature negative pressure dehydration treatment; (3) The product of step (2) is reacted with a catalyst to produce lactide. During the reaction, measures are taken to remove the water produced in the reaction.

2. The method according to claim 1, characterized in that, The concentration of the lactic acid solution is 20wt%-80wt%.

3. The method according to claim 1, characterized in that, The heating reaction in step (1) is carried out at a temperature of 80-120℃ for 1-3 hours.

4. The method according to claim 1, characterized in that, Step (1) The reaction of dehydrating lactic acid to produce lactic acid dimer is carried out in a reactor connected to a drainage device, which is used to control the water content of the substrate in the reaction system to be maintained at 20-50%.

5. The method according to claim 4, characterized in that, The reaction of lactic acid dehydration to produce lactic acid dimer in step (1) is carried out in a reactor connected to a vacuum drainage system, and the pressure of the reactor is controlled at 60-85 kPa.

6. The method according to claim 1, characterized in that, The low-temperature negative pressure dehydration process involves removing water from the lactic acid dimer under conditions of controlled temperature of 40-60℃ and vacuum of 4-20Kpa.

7. The method according to claim 1, characterized in that, The low-temperature negative pressure dehydration treatment dehydrates the substrate to a moisture content of less than 8%.

8. The method according to claim 1, characterized in that, Step (3) is achieved using a fixed bed or catalytic distillation column reaction.

9. The method according to claim 1, characterized in that, The catalyst is a zeolite catalyst.

10. The method according to claim 9, characterized in that, The catalyst is selected from at least one of HY, H-USY, SAPO-34, titanium silicate molecular sieve and H-Beta.

11. The method according to claim 1, characterized in that, The measures taken to remove water generated in step (3) are solvent removal and / or air stripping removal.

12. The method according to claim 11, characterized in that, The solvent is selected from at least one of benzene, toluene, xylene, methyl isobutyl ketone, cyclohexanone, and cyclohexane.

13. The method according to claim 8, characterized in that, The reaction process in step (3) is preferably carried out by a catalytic distillation column: the catalyst is packed in the middle reaction section of the catalytic distillation column, the product of step (2) is introduced into the catalytic distillation column from the top, the organic solvent or reaction inert gas is introduced from the bottom of the catalytic distillation column, and after passing through the middle reaction section, the water removed by the reaction is carried out from the top of the column, and the reaction product containing lactide is taken out from the bottom of the column.

14. The method according to claim 13, characterized in that, In step (3), the volume ratio of the reaction raw materials to the organic solvent entering the catalytic distillation column per unit time is 1:3-10.

15. The method according to claim 1, characterized in that, The ratio of the mass flow rate of the reactant entering the catalytic distillation column to the mass of the catalyst is 1-10 h⁻¹. -1 .

16. The method according to claim 1, characterized in that, The operating pressure of the catalytic distillation column is 0.05-0.3 MPa; the temperature of the intermediate reaction section of the catalytic distillation column is 100℃-140℃.

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