Method for preparing lactate from mixed plastic containing polylactic acid

Abstract

The invention relates to the technical field of lactate preparation, in particular to a method for preparing lactate from mixed plastic containing polylactic acid. According to the invention, polylactic acid-containing mixed plastic, water and a metal compound are used as raw materials, and polylactic acid is degraded at high temperature under an anaerobic condition and reacts with water and the metal compound to obtain the lactate. According to the method disclosed by the invention, the high-purity lactate can be prepared by taking the waste polylactic acid as a raw material under the condition that only a small amount of metal compounds are added, a separation step is not needed, and a fermentation product lactic acid or calcium lactate is not taken as the raw material, so that the raw material and post-treatment cost is greatly reduced; meanwhile, selective resource utilization of polylactic acid in the mixed waste plastic is achieved, and the pollution problem caused by incomplete degradation of the waste polylactic acid and the problem that polylactic acid plastic in the mixed waste plastic is difficult to separate are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lactate preparation, and particularly relates to a method for preparing lactate by using a mixed plastic containing polylactic acid. Background Art

[0002] At present, the fertilizers used to supplement trace elements for plants in agriculture are mostly metal oxides or inorganic salts, and there are also organic salts such as amino acid salts and EDTA salts; however, inorganic fertilizers mostly have problems such as being insoluble in water and poor plant absorption, while organic fertilizers are easy to absorb but generally have a high price.

[0003] Lactate is a chemical substance used in agricultural trace element supplements and can simultaneously provide a carbon source for plants, and the plant absorption is good. At present, lactate is usually prepared from lactic acid. The methods for producing lactate are divided into two types. Taking zinc lactate as an example, one is to directly react lactic acid with zinc oxide / zinc hydroxide; the other is to carry out a metathesis reaction using calcium lactate and zinc sulfate. Both of these methods require the use of lactic acid or calcium lactate produced by fermentation. Commercial lactic acid basically comes from the fermentation method, that is, specific strains convert raw materials into lactic acid at a certain temperature and pH. The raw materials are usually sugars (such as glucose, fructose, maltose) or starchy raw materials (such as corn, wheat, potatoes). After fermentation is completed, the commonly used separation method is the calcium neutralization method, that is, adding calcium carbonate to react with lactic acid to generate calcium lactate, and then acidifying with sulfuric acid to obtain lactic acid. First, the temperature, pH value and substrate concentration need to be strictly controlled during the fermentation process, otherwise the fermentation effect may be poor or even the strain may die; second, the calcium neutralization method will cause the accumulation of calcium lactate crystals, inhibiting the growth of microorganisms; in addition, it is also necessary to provide nutrients such as nitrogen sources and vitamins essential for the strains, and the cost of these nutrients is usually not low, which also increases the production cost. These factors all lead to a high production cost of lactic acid, and further lead to a high cost of lactate.

[0004] Existing technologies for preparing lactate from polylactic acid, such as Patent Publication No. CN118308742A discloses a method for preparing acetate and pyruvate from waste polylactic acid plastics. First, the polylactic acid particles are hydrothermally degraded with an alkaline solvent at 60°C to obtain an aqueous lactate solution, and then the aqueous lactate solution is electro-oxidized to obtain acetate and pyruvate. Patent Publication No. CN109761798A discloses a method for preparing lactate, which alkali-decomposes waste polylactic acid and then adjusts the pH ≤ 7 to obtain lactate.

[0005] It can be seen that the above-mentioned existing technologies all first hydrolyze polylactic acid with high-pH alkali liquor. The technical defects are as follows: on the one hand, a large amount of high-pH alkali liquor needs to be input, which is likely to cause equipment corrosion. At the same time, it is difficult to separate and purify the degraded polylactic acid, and the feasibility is not high; on the other hand, the existing technologies only achieve the degradation of polylactic acid or the raw materials in the process of producing polylactic acid, and cannot process mixed plastics, which requires sorting of plastics and is inconvenient to operate. Moreover, after the existing technologies degrade polylactic acid with alkali liquor, secondary treatment is still required to obtain lactate, which is not only cumbersome in operation, but also increases the cost additionally, and is not conducive to the industrial preparation of lactate.

[0006] In addition, to solve the problem of waste plastics, biodegradable plastics have received extensive attention in recent years. Among them, polylactic acid (PLA) has excellent biocompatibility and biodegradability, and the raw materials can be obtained from fermentation products, which is an important polymer material. PLA can be completely degraded under optimized composting conditions. However, under natural conditions, the degradation process of PLA is greatly affected by environmental conditions, the degradation rate is slow, it may take several years or even decades, and it usually does not degrade completely, but will form microplastics, which will have an impact on the environment and biological health. Summary of the Invention

[0007] In view of the above deficiencies of the existing technologies, the present invention provides a method for preparing lactate from a mixed plastic containing polylactic acid. The present invention uses a mixed plastic containing polylactic acid, water and a metal compound as raw materials. Under anaerobic conditions, by simultaneously degrading polylactic acid at high temperature, the reaction with water and the metal compound is achieved in one step to obtain lactate. The present invention can prepare lactate from waste polylactic acid, prepare high-purity lactate with only a small amount of metal compound input, and does not require a separation step. Instead of using lactic acid or calcium lactate from fermentation products as raw materials, the raw material and post-treatment costs are greatly reduced; at the same time, the selective resource utilization of polylactic acid in mixed waste plastics is realized, and the pollution problem caused by incomplete degradation of waste polylactic acid and the problem of difficult separation of polylactic acid plastics in mixed waste plastics are overcome.

[0008] To achieve the above object, the technical solution of the present invention is as follows:

[0009] A method for preparing lactate from a mixed plastic containing polylactic acid, comprising the following steps:

[0010] Under anaerobic conditions, a mixed plastic containing polylactic acid, water, and a metal compound are mixed and subjected to a degradation reaction at 170 °C to 300 °C. At this time, polylactic acid degrades and reacts with water and the metal compound. Too low a temperature will make it difficult for the degradation reaction of polylactic acid to proceed and reduce the yield; high temperature will not cause a reduction in yield, but too high a temperature will lead to unnecessary resource waste and higher requirements for equipment and processes. For example, high temperature leads to greater pressure and insufficient deoxygenation leading to oxidation problems. The one-step method realizes the preparation of lactate. Among them, the cation of the metal compound is selected from +2 or +3 valence cations.

[0011] Preferably, the metal is selected from Mg, Al, Ca, Fe, Ni, Cu, or Zn.

[0012] Preferably, the metal compound is selected from metal oxides, metal hydroxides, or metal carbonates.

[0013] Preferably, the number-average molecular weight Mn of polylactic acid ≤ 30,000 g / mol. The degradation reaction is a process of reducing the molecular weight of polylactic acid. Therefore, the lower limit of the molecular weight is relatively low, and low molecular weight will not have an impact, but too high a molecular weight will lead to insufficient degradation.

[0014] Preferably, based on the repeating unit of polylactic acid, the molar ratio of polylactic acid to water is 1:0.1 to 10. Too much or too little water content will both lead to a reduction in reaction efficiency, but the impact is relatively small. Since water is consumed and generated in equal amounts during the reaction, the addition amount has a relatively small impact on the reaction, so the range is relatively wide.

[0015] Preferably, based on the repeating unit of polylactic acid, the molar ratio of polylactic acid to divalent metal compounds (such as Mg, Ca, Fe(II), Ni, Cu, Zn metal compounds) is 2:0.5 - 1, and the molar ratio of polylactic acid to trivalent metal compounds (such as Al, Fe(III) metal compounds) is 3:0.5 - 1, with polylactic acid based on the repeating unit. Too little metal compound will result in a relatively small content of lactate in the product and a relatively large content of lactic acid; too much metal compound will lead to resource waste or increased cost.

[0016] Preferably, the degradation reaction time is 0.5 h to 8 h. Too short a time will lead to insufficient degradation; too long a time will waste energy.

[0017] Preferably, the polylactic acid is selected from waste polylactic acid.

[0018] Preferably, in the mixed plastic containing polylactic acid, polylactic acid ≥ 20 wt%. The method of the present invention realizes the purpose of only degrading polylactic acid in the mixed plastic.

[0019] Compared with the prior art, the beneficial effects of the present invention:

[0020] 1. The present invention uses a mixed plastic containing polylactic acid, water, and a metal compound as raw materials to prepare lactate. Research shows that at room temperature, polylactic acid is insoluble in water, has a high molecular weight, and is inert, and cannot react with poorly soluble metal compounds. However, after heating to a temperature ≥170 °C, polylactic acid melts and degrades, first reacts with water to form lactic acid, and then lactic acid reacts with the metal compound to form lactate and water.

[0021] In addition, in the present invention, water is used as a reactant rather than a solvent. If water is replaced with other substances, lactate cannot be obtained as a product.

[0022] 2. The raw materials of the present invention are easily available, and no organic solvents or catalysts are used in the reaction process. The reaction process is simple and no post-treatment is required. It can achieve rapid depolymerization of polylactic acid under mild industrial conditions and efficiently obtain lactate, which is suitable for large-scale production.

[0023] 3. The present invention realizes the degradation and resource utilization of waste polylactic acid, can avoid environmental pollution by waste plastics, improve economic efficiency, and reduce carbon emissions. In addition, compared with the prior art, the present invention uses waste polylactic acid as a raw material instead of lactic acid or calcium lactate obtained by fermentation, overcomes the cost problem caused by fermentation condition constraints, and realizes the preparation of lactate at low cost.

[0024] 4. The lactate obtained by the present invention can be directly used as an agricultural trace element supplement and can be used as an easily absorbable organic fertilizer. In addition, the method provided by the present invention can use oxides, hydroxides, or carbonates of various metals as raw materials and produce corresponding lactates, which is suitable for the preparation of various trace element supplements and has good universality.

[0025] 5. The method provided by the present invention has the characteristics of high yield and few side reactions. In addition, the present invention is less interfered by other components, can selectively degrade polylactic acid under mixed plastic conditions, and obtain high-purity products. The mixed plastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polycarbonate (PC), copolymers of butylene adipate and butylene terephthalate (PBAT), and nylon.

[0026] 6. The present invention uses a low amount of metal compound, can use a metal compound insoluble in water to carry out the reaction in a neutral environment, is not easy to damage the equipment, only degrades polylactic acid into lactate in the mixed plastic, and the product does not require post-treatment, has little environmental pollution, and has cost advantages and high feasibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the reaction mechanism of the present invention.

[0028] Figure 2In it, a is the 1 1H NMR spectrum of zinc lactate in Example 1, and b is the 13 13C NMR spectrum of zinc lactate in Example 1.

[0029] Figure 3 It is the conversion rate and selectivity diagram of polylactic acid at different reaction times in Example 6.

[0030] Figure 4 It is the 1 1H NMR spectrum of zinc lactate in Example 6. Detailed implementation manners

[0031] The following describes the detailed implementation manners of the present invention in detail, but it should be understood that the protection scope of the present invention is not limited by the detailed implementation manners. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention. The experimental methods described in the embodiments of the present invention are all conventional methods unless otherwise specified.

[0032] The following experimental methods and detection methods are all conventional methods unless otherwise specified; the following reagents and raw materials are all commercially available unless otherwise specified.

[0033] Considering the technical problem that the degradation rate of polylactic acid is slow under natural conditions, microplastics will be formed due to incomplete degradation, and then the environment will be polluted, the present invention starts from waste polylactic acid and uses waste polylactic acid as the raw material to solve the problem of environmental pollution while realizing resource recycling.

[0034] Considering that lactate in the prior art is usually prepared from lactic acid, and lactic acid produced by fermentation or calcium lactate is required, and fermentation requires strict control of temperature, pH value and substrate concentration, and the calcium neutralization method will produce the accumulation of calcium lactate crystals to inhibit the growth of microorganisms, and nutrients such as nitrogen source and vitamins necessary for the strain need to be provided. These factors all lead to a high production cost of lactic acid, and further lead to a high cost of lactate. The present invention uses waste polylactic acid as the raw material instead of lactic acid or calcium lactate produced by fermentation, effectively reducing the cost and enabling the industrial production of lactate.

[0035] Considering the technical defects existing in the alkaline degradation of polylactic acid and the two-step method for preparing lactate in the prior art, on the one hand, the present invention uses the hydrothermal method to degrade polylactic acid, and on the other hand, under high temperature conditions, the reaction of lactic acid with metal compounds is realized, that is, the preparation of lactate is realized under one-step and neutral conditions, which is more convenient for the industrial production of lactate.

[0036] In addition, in the method for preparing lactate using polylactic acid provided by the present invention, the degradation of polylactic acid in the mixed plastic can be achieved. Therefore, even if impurities are incorporated, it does not affect the preparation of lactate, further improving the feasibility of industrialization.

[0037] It can be seen that compared with the prior art, the present invention provides a completely new method for preparing lactate. Using the preparation method of the present invention, the purpose of preparing lactate at low cost is achieved, and the industrial production of lactate is also realized, which has positive significance.

[0038] The technical solution of the present invention will be further explained and illustrated by the following examples, which are specifically as follows:

[0039] Example 1

[0040] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0041] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water, and 0.204 g of zinc oxide to a 100 mL glass pressure-resistant bottle. After purging with nitrogen, place the pressure-resistant bottle in an oil bath at 180 °C and react for 2 h. Take out the glass pressure-resistant bottle and naturally cool it to room temperature. Add water to dissolve it fully, filter, and the filtrate is dried to obtain the product zinc lactate. The yield is 98.4%.

[0042] Example 2

[0043] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0044] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water, and 0.101 g of magnesium oxide to a 100 mL glass pressure-resistant bottle. After purging with nitrogen, place the pressure-resistant bottle in an oil bath at 180 °C and react for 2 h. Take out the glass pressure-resistant bottle and naturally cool it to room temperature. Add water to dissolve it fully, filter, and the filtrate is dried to obtain the product magnesium lactate. The yield is 98.2%.

[0045] Example 3

[0046] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0047] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water, and 0.250 g of calcium carbonate to a 100 mL glass pressure-resistant bottle. After purging with nitrogen, place the pressure-resistant bottle in an oil bath at 180 °C and react for 4 h. Take out the glass pressure-resistant bottle and naturally cool it to room temperature. Add water to dissolve it fully, filter, and the filtrate is dried to obtain the product calcium lactate. The yield is 96.8%.

[0048] Example 4

[0049] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0050] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water and 0.204 g of zinc oxide to a 100 mL micro high-pressure reactor. After purging with nitrogen, heat the reactor to 300 °C and react for 0.5 h. After completion, cool it naturally to room temperature, add water to dissolve it fully, filter, and the filtrate is dried to obtain the product zinc lactate. The yield is 99.8%.

[0051] Example 5

[0052] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0053] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water and 0.2 g of aluminum hydroxide to a 100 mL micro high-pressure reactor. After purging with nitrogen, heat the reactor to 170 °C and react for 8 h. After completion, cool it naturally to room temperature, add water to dissolve it fully, filter, and the filtrate is dried to obtain the product zinc lactate. The yield is 95.4%.

[0054] Example 6

[0055] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0056] Add 0.36 g of mixed plastic powder (containing 65 wt% polylactic acid, 5 wt% HDPE, 5 wt% PP, 5 wt% PS, 5 wt% PET, 5 wt% PC, 5 wt% PBAT, 5 wt% nylon 6, polylactic acid Mn = 22000 g / mol), 1.8 g of water and 0.204 g of zinc oxide to a 100 mL glass pressure-resistant bottle. After purging with nitrogen, place the glass pressure-resistant bottle in an oil bath at 180 °C and react for 4 h. Take out the glass pressure-resistant bottle and cool it naturally to room temperature, add water to dissolve it fully, filter, and the filtrate is dried to obtain the product zinc lactate. The yield is 94.8%.

[0057] Comparative Example 1

[0058] A method for preparing lactate using a mixed plastic containing polylactic acid, comprising the following steps:

[0059] Add 0.36 g of polylactic acid powder (Mn = 22000 g / mol), 1.8 g of water and 0.204 g of zinc oxide to a 100 mL glass pressure-resistant bottle. After purging with nitrogen, place the pressure-resistant bottle in an oil bath at 140 °C and react for 2 h. Take out the glass pressure-resistant bottle and cool it naturally to room temperature, add water to dissolve the product fully, filter, and the filtrate is dried to obtain the product zinc lactate. The yield is 50%.

[0060] The zinc lactate of Example 1 and Example 7 of the present invention was used for research, and the research results are as follows:

[0061] Figure 1 The results show that the method of the present invention achieves the purpose of preparing various lactates using various metal compounds as raw materials.

[0062] Figure 2 The results show that after degrading polylactic acid and reacting with water and zinc oxide, a high-purity zinc lactate product was obtained.

[0063] Figure 3 The results show that using the method of the present invention with a mixed plastic mainly composed of polylactic acid as the raw material can also achieve efficient degradation of polylactic acid.

[0064] Figure 4 The results show that using the method of the present invention to degrade and react the mixed plastic can also obtain a high-purity lactate product.

[0065] Obviously, those skilled in the art can make various changes and deformations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and deformations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and deformations. The above-described embodiments are only preferred embodiments given to fully illustrate the present invention, and the scope of protection is not limited thereto. Equivalent substitutions or transformations made by those skilled in the art on the basis of the present invention are all within the scope of protection of the present invention, and the scope of protection of the present invention is subject to the claims.

Claims

1. A method for preparing lactate using a mixed plastic containing polylactic acid, characterized in that It includes the following steps: Under anaerobic conditions, a mixed plastic containing polylactic acid, water and a metal compound are mixed and subjected to a degradation reaction at 170°C to 300°C. At this time, the polylactic acid degrades and reacts with water and the metal compound to obtain a lactate; Among them, the cation of the metal compound is selected from +2-valent or +3-valent cations.

2. The method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, The metal of the metal compound is selected from Mg, Al, Ca, Fe, Ni, Cu or Zn.

3. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, The metal compound is selected from metal oxides, metal hydroxides or metal carbonates.

4. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, Based on the repeating unit of polylactic acid, the molar ratio of polylactic acid to water is 1:0.1 to 10.

5. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, Based on the repeating unit of polylactic acid, the molar ratio of polylactic acid to the divalent metal compound is 2:0.5 - 1.

6. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, Based on the repeating unit of polylactic acid, the molar ratio of polylactic acid to the trivalent metal compound is 3:0.5 - 1.

7. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, The degradation reaction time is 0.5 h to 8 h.

8. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, The polylactic acid is selected from waste polylactic acid.

9. A method for preparing lactate using a mixed plastic containing polylactic acid according to claim 1, characterized in that, In the mixed plastic containing polylactic acid, the content of polylactic acid is ≥20 wt%.

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