A method for preparing lactide by degrading polylactic acid by a dry steam method
Direct degradation of polylactic acid by dry steam method solves the problems of low yield and complex process of polylactic acid depolymerization in the prior art, and achieves efficient, green and low energy consumption lactide preparation, reducing economic and environmental costs.
Patent Information
- Application Number
- CN202310292287.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-03-23
- Publication Date
- 2025-06-03
- Estimated Expiration
- 2043-03-23
AI Technical Summary
In the prior art, the method of depolymerizing polylactic acid into lactide has problems such as low yield, complex process, needing to add catalysts or additives, and harsh reaction conditions, resulting in high economic and environmental costs.
By using the dry steam method, the dry steam pressure is controlled at 0.31~1.72 Mpa, the reaction temperature is 160~210 °C, and the reaction time is 60~220 minutes, the polylactic acid is directly degraded to obtain lactide, without using any additives, catalysts or organic solvents.
The production of lactide with high efficiency, green and low energy consumption is achieved. The yield is high, the reaction conditions are mild and pollution-free, reducing production costs and environmental impacts.
Smart Images
Figure CN116375679B_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of renewable plastic waste resource utilization, and particularly relates to a method for preparing lactide by degrading polylactic acid by a dry steam method. Background Art
[0002] Although polylactic acid is a biodegradable bio-based plastic, there are still potential resource and environmental problems in the production process and degradation process of polylactic acid. Currently, the main polylactic acid production process uses food-source organic substances such as corn starch or glucose as reaction raw materials, ferments lactic acid by a biological method, dehydrates and cyclizes lactic acid to generate lactide, and finally catalyzes the ring-opening polymerization of lactide to generate polylactic acid. At present, the main polylactic acid chemical recycling strategy is to depolymerize polylactic acid by hydrolysis and alcoholysis methods to prepare high-value-added monomers (lactic acid, lactate, and alkyl lactate), but the obtained monomers still need to be further converted into lactide, and then lactide is polymerized again to generate polylactic acid. The economic and environmental costs of the recycled polylactic acid obtained through this recycling strategy are relatively high; in addition, the high technical barriers of the production process for preparing lactide from lactic acid also limit the application and popularization of related technologies. Therefore, directly depolymerizing polylactic acid to prepare lactide will be a more economical and green polylactic acid recycling strategy.
[0003] Currently, there is little research on depolymerizing polylactic acid to lactide, and only a few studies have used pyrolysis methods to depolymerize polylactic acid to obtain lactide. At relatively high temperatures of 250-380 °C, polylactic acid is pyrolyzed alone, and the lactide yield is less than 20%. The low yield has become the main limitation of this method. To solve the problem of low yield, some research reports have used adding catalysts and solvents to depolymerize polylactic acid to lactide under relatively harsh reaction steps and conditions. For example, polylactic acid is dissolved in solvents (PPG400, PPG600, PPG1000, PPG2000, PEG400, PEG600, etc.), and a metal catalyst is added to carry out the depolymerization reaction of polylactic acid. There are also research reports on catalytically degrading polylactic acid by mixing polylactic acid with strong bases under high-temperature and high-vacuum conditions. All these methods have cumbersome processes, require external catalysts or additives or organic solvents, and have harsh reaction conditions. Therefore, how to depolymerize polylactic acid to prepare lactide in a green, low-carbon, simple, and efficient manner has become a challenging and meaningful topic. Summary of the Invention
[0004] The purpose of the present invention is to provide a method for preparing lactide by degrading polylactic acid by a dry steam method, which is green, low-energy-consuming, carbon-free, has mild reaction conditions, no pollution, and no catalyst.
[0005] The object of the present invention can be achieved by the following technical solutions: A method for preparing lactide by degrading polylactic acid using dry steam. By using the dry steam method, the dry steam pressure is controlled at 0.31 - 1.72 Mpa, the reaction temperature is 160 - 210 °C, and under the condition of a reaction time of 60 - 220 minutes, polylactic acid is directly degraded to obtain lactide.
[0006] The present invention does not use any additives, catalysts, or organic solvents, and does not require any pretreatment steps. The "one-pot method" is used to directly depolymerize polylactic acid to prepare lactide.
[0007] The present invention relates to a method for directly converting polylactic acid into lactide using dry steam, including: controlling water in a reactor to create a dry steam atmosphere, and then under the condition of dry steam at 160 °C - 210 °C, after polylactic acid directly reacts for 60 - 220 minutes, lactide is generated.
[0008] Preferably, the dry steam pressure is controlled at 0.31 - 0.64 Mpa.
[0009] Preferably, the reaction temperature is 170 - 190 °C.
[0010] Preferably, the reaction time is 150 - 220 minutes.
[0011] Preferably, when the reaction temperature is 190 °C and the reaction time is 150 minutes, the highest lactide yield of 46% is obtained.
[0012] Preferably, the initial addition amount of polylactic acid is 0.6 - 2.0 g / Mpa.
[0013] More preferably, the initial addition amount of polylactic acid is 0.83 - 1.57 g / Mpa.
[0014] Preferably, the polylactic acid refers to a polyester polymer obtained by polymerizing lactic acid as the main raw material.
[0015] Alternatively, the polylactic acid refers to a polylactic acid plastic product.
[0016] The polylactic acid does not require any pulverization and drying treatment.
[0017] Preferably, the dry steam refers to a reaction field with high enthalpy, low density, and low dielectric constant without liquid water below the saturated vapor pressure equilibrium curve.
[0018] Preferably, polylactic acid and water for producing dry steam are respectively placed in the first reaction chamber and the second reaction chamber of the reaction kettle, so that the water for producing dry steam has no contact with polylactic acid. The dry steam generated in the second reaction chamber enters the first reaction chamber along the connecting channel to form a dry steam atmosphere, and polylactic acid undergoes a degradation reaction in the dry steam atmosphere. After the reaction, it is cooled to room temperature.
[0019] The first step of the formation of lactide from polylactic acid is the selective protonation of the alkyl bond and acyl bond on polylactic acid to break, and the second step is the partial cyclization and breakage to form lactide. Therefore, how to selectively protonate the non-adjacent alkoxy bond and acyloxy bond on polylactic acid is the key to the depolymerization of polylactic acid to produce lactide. Dry steam refers to a reaction field with high enthalpy, low density, and low dielectric constant without liquid water below the saturated vapor pressure equilibrium curve. Analyzing from the structure of water and the intermolecular interaction, the number of hydrogen bonds between water molecules in dry steam decreases, and it is easier to provide protons. By regulating the dry steam pressure, changing the hydrogen bond force of water molecules, water molecules provide protons to selectively protonate the alkoxy bond and acyloxy bond on polylactic acid. However, there has been no report on the study of preparing lactide by degrading polylactic acid with dry steam so far.
[0020] From the perspective of practical engineering, compared with the hydrolysis method and alcoholysis method for depolymerizing polylactic acid to lactic acid, which have been studied more, the dry steam method of the present invention can directly degrade polylactic acid to lactide, avoiding the energy, equipment, and capital investment required for the link of preparing lactide by lactic acid dehydration cyclization. Moreover, the reaction conditions are milder and no additional catalyst is needed. In addition, dry steam has better energy exchange ability, effectively avoiding problems such as deposition, corrosion, and shortened service life of boilers or other heat exchangers. The results of life cycle research show that the energy load of producing lactide by depolymerizing polylactic acid is also much smaller than that of producing lactide by cyclizing lactic acid produced from corn. Therefore, under the dry steam atmosphere, the direct conversion of polylactic acid to lactide can produce good social, environmental, and economic benefits. In addition, compared with the method of depolymerizing polylactic acid to lactide by pyrolysis, the reaction conditions for preparing lactide by treating polylactic acid with dry steam are significantly milder, the yield is high, and no external additives or catalysts are needed. It is a new method for the resource recycling of polylactic acid that is green, environmentally friendly, carbon-negative, cheap, simple, and easy to industrialize.
[0021] Compared with the prior art, the present invention has the following advantages:
[0022] 1. The present invention uses polylactic acid as a raw material and adopts the dry steam method to degrade polylactic acid to produce lactide, which has the characteristics of being green, low energy consumption, carbon-free emission, mild reaction conditions, pollution-free, and catalyst-free, and is green and sustainable;
[0023] 2. Compared with the existing methods for preparing lactide, the present invention does not require land and fertilizers in the production process, does not cause the problem of "competing with people for food", and the energy load required for producing lactide from polylactic acid is low;
[0024] 3. Compared with the existing methods for treating polylactic acid by pyrolysis, hydrolysis, and alcoholysis, the present invention treats polylactic acid by a simple dry steam method without any additional catalysts and additives, without adding any additional organic solvents, with mild reaction conditions, and can obtain lactide, a high-value-added compound;
[0025] 4. The method of the present invention is also characterized by simple process, high atom economy, low energy consumption, no secondary pollution, environmental friendliness, and very low cost, and has objective environmental, economic, and social benefits;
[0026] 5. Through the green and environment-friendly disposal of polylactic acid, the present invention avoids the carbon emission process during the natural degradation of polylactic acid and also obtains lactide, contributing to global CO 2 emission reduction and having obvious economic, environmental, and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Figure 1 is a comparison diagram of the method of the present invention and the prior art method;
[0028] Figure 2 is the high performance liquid chromatography characterization spectrum of Example 4 of the present invention;
[0029] Figure 3 is the high performance liquid chromatography characterization spectrum of Comparative Example 2;
[0030] Figure 4 is the gas chromatography - mass spectrometry characterization spectrum of Example 4 of the present invention;
[0031] Figure 5 is the gas chromatography - mass spectrometry characterization spectrum of Comparative Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0032] The present invention will be described in detail below with reference to the drawings and specific embodiments. The following embodiments are implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.
[0033] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used, unless otherwise specified, are all conventional materials, reagents, methods, and instruments in the art, and those skilled in the art can obtain them through commercial channels.
[0034] A method for degrading polylactic acid into lactide using dry steam. Under the conditions of controlling the dry steam pressure at 0.31 - 1.72 Mpa, the temperature at 160 - 210 °C, and the reaction time at 60 - 220 minutes, polylactic acid is degraded into lactide by a one-pot method using dry steam. In the present invention, the yield of lactide is optimized by changing the dry steam pressure, the addition amount of polylactic acid, the reaction temperature, and the reaction time.
[0035] In Examples 1 - 8, polylactic acid was depolymerized by the dry steam method, and in Comparative Examples 1 and 2, polylactic acid was depolymerized by the pyrolysis method to prepare lactide. As Figure 1 shown, compared with the techniques of degrading polylactic acid to produce lactide by hydrolysis and alcoholysis methods, the present invention has the significant advantage of directly converting into lactide; compared with the method of degrading polylactic acid to produce lactide by the pyrolysis method, the one-pot method of the present invention has a simple process, milder reaction conditions, higher yield, does not require any catalyst and additive, and does not require the addition of any organic solvent. It is a new method for the resource recycling of polylactic acid that is green, environmentally friendly, carbon-negative, inexpensive, simple, and easy to industrialize.
[0036] Example 1
[0037] In this example, an experimental study on the degradation of polylactic acid into lactide was carried out. First, a small amount of water capable of generating dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.48 Mpa, the initial addition amount of polylactic acid was 0.83 g / Mpa, and the reaction was carried out at 160 °C for 220 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide using high-performance liquid chromatography (HPLC).
[0038] Example 2
[0039] An experimental study on the degradation of polylactic acid into lactide was carried out in this example. First, a small amount of water capable of generating dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.31 Mpa, the initial addition amount of polylactic acid was 1.13 g / Mpa, and the reaction was carried out at 170 °C for 220 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide using high-performance liquid chromatography (HPLC).
[0040] Example 3
[0041] An experimental study on the degradation of polylactic acid into lactide was carried out in this example. First, a small amount of water capable of generating dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.50 Mpa, the initial addition amount of polylactic acid was 1.06 g / Mpa, and the reaction was carried out at 180 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide using high-performance liquid chromatography (HPLC).
[0042] Example 4
[0043] An experimental study on the degradation of polylactic acid to convert it into lactide was carried out in this example. First, a small amount of water capable of producing dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.51 Mpa, the initial addition amount of polylactic acid was 1.06 g / Mpa, and the reaction was carried out at 190 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high-performance liquid chromatography (HPLC).
[0044] The high-performance liquid chromatography characterization spectrum of lactide in this example is as Figure 2 shown, and the characterization spectrum of gas chromatography-mass spectrometry is as Figure 4 shown.
[0045] Example 5
[0046] An experimental study on the degradation of polylactic acid to convert it into lactide was carried out in this example. First, a small amount of water capable of producing dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.51 Mpa, the initial addition amount of polylactic acid was 1.57 g / Mpa, and the reaction was carried out at 190 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high-performance liquid chromatography (HPLC).
[0047] Example 6
[0048] An experimental study on the degradation of polylactic acid to convert it into lactide was carried out in this example. First, a small amount of water capable of producing dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.64 Mpa, the initial addition amount of polylactic acid was 1.56 g / Mpa, and the reaction was carried out at 190 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high-performance liquid chromatography (HPLC).
[0049] Example 7
[0050] An experimental study on the degradation of polylactic acid to convert it into lactide was carried out in this example. First, a small amount of water capable of producing dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.52 Mpa, the initial addition amount of polylactic acid was 1.08 g / Mpa, and the reaction was carried out at 200 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high-performance liquid chromatography (HPLC).
[0051] Example 8
[0052] An experimental study on the degradation of polylactic acid to convert it into lactide was carried out in this example. First, a small amount of water capable of producing dry steam was added to the reaction kettle. Under the dry steam atmosphere of 0.54 Mpa, the initial addition amount of polylactic acid was 1.13 g / Mpa, and the reaction was carried out at 210 °C for 60 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high-performance liquid chromatography (HPLC).
[0053] Comparative Example 1
[0054] In this comparative example, an experimental study on the pyrolysis conversion of polylactic acid to lactide was carried out. First, the same weight of polylactic acid as in Example 4 was added to the reaction kettle, and the polylactic acid was directly pyrolyzed at 190 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high performance liquid chromatography (HPLC).
[0055] Comparative Example 2
[0056] In this comparative example, an experimental study on the pyrolysis conversion of polylactic acid to lactide was carried out. First, the same weight of polylactic acid as in Example 4 was added to the reaction kettle, and the polylactic acid was directly pyrolyzed at 300 °C for 150 minutes. After the reaction, it was cooled to room temperature, and the liquid-phase product was qualitatively and quantitatively analyzed for lactide by high performance liquid chromatography (HPLC).
[0057] The high performance liquid chromatography characterization map of lactide in this comparative example is as Figure 3 shown, and the gas chromatography-mass spectrometry characterization map is as Figure 5 shown.
[0058] Table 1 Experimental results of depolymerizing polylactic acid by dry steam method to prepare lactide
[0059] Reaction term Reaction method Dry steam pressure (Mpa) Temperature (°C) Lactic acid addition amount (g / Mpa) Time (minutes) Lactide yield (%) Example 1 Dry steam method 0.48 160 0.83 220 21.9 Example 2 Dry steam method 0.31 170 1.13 220 42.0 Example 3 Dry steam method 0.50 180 1.06 150 44.0 Example 4 Dry steam method 0.51 190 1.06 150 46.0 Example 5 Dry steam method 0.51 190 1.57 150 43.4 Example 6 Dry steam method 0.64 190 1.56 150 40.5 Example 7 Dry steam method 0.52 200 1.08 150 38.5 Example 8 Dry steam method 0.54 210 1.13 60 33.6 Comparative example 1 Pyrolysis method n / a 190 1.06 150 n / a Comparative example 2 Pyrolysis method n / a 300 1.06 150 3.3
[0060] Note: The calculation method of lactide yield is as follows: Lactide yield = (mass of generated lactide / theoretically generated mass of lactide) × 100 wt%.
[0061] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention should be within the protection scope of the present invention.
Claims
1. A method for preparing lactide by degrading polylactic acid using dry steam, characterized in that, using the dry steam method, controlling the dry steam pressure at 0.31 - 0.64 Mpa, the reaction temperature at 160 - 210 °C, and the reaction time at 60 - 220 minutes, directly degrade polylactic acid to obtain lactide; the initial addition amount of the polylactic acid is 0.6 - 2.0 g / Mpa.
2. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the reaction temperature is 190 °C and the reaction time is 150 minutes.
3. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the initial addition amount of the polylactic acid is 0.83 - 1.57 g / Mpa.
4. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the water for producing dry steam has no contact with the polylactic acid.
5. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the polylactic acid is a polyester polymer obtained by polymerizing lactic acid as the main raw material.
6. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the polylactic acid is a polylactic acid plastic product.
7. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, the dry steam is a reaction field without liquid water below the saturated vapor pressure equilibrium curve.
8. The method for preparing lactide by degrading polylactic acid using dry steam according to claim 1, characterized in that, put the polylactic acid and the water for producing dry steam into the first reaction chamber and the second reaction chamber of the reaction kettle respectively. The dry steam generated in the second reaction chamber enters the first reaction chamber along the connecting channel to form a dry steam atmosphere, and the polylactic acid undergoes a degradation reaction in the dry steam atmosphere. After the reaction, it is cooled to room temperature.