Method for catalyzing degradation of polylactic acid by using modified activated carbon

Abstract

According to the technical scheme, the method comprises the following steps that 1, activated carbon is added into water, heating and stirring are conducted, suction filtration is conducted after stirring is conducted, then solids are collected and dried, and activated AC is obtained; 2, adding all AC into the HNO3 aqueous solution, heating and stirring, and condensing and refluxing for 3-15 hours; after the reflux is finished, washing a product to be neutral, and drying overnight to obtain AC-H; 3, transferring AC-H into a NaOH aqueous solution, stirring at normal temperature, and then washing to be neutral to obtain AC-H-Na; 4, polylactic acid, AC-H-Na and absolute methanol are added into a reaction kettle, the temperature is increased to the specified temperature in the nitrogen environment for a reaction, and methyl lactate is obtained; the method adopts nitric acid and sodium hydroxide step-by-step modified activated carbon as a catalyst to catalyze degradation of polylactic acid, has high catalytic efficiency, and the catalyst is easy to recover, and reduces reaction energy consumption at the same time; belongs to the technical field of polymer degradation.

Description

Technical Field

[0001] The present invention discloses a method for the degradation of polylactic acid, specifically, a method for the degradation of polylactic acid catalyzed by modified activated carbon, belonging to the technical field of polymer degradation. Background Art

[0002] As one of the most representative biodegradable polymers, polylactic acid (PLA) has been widely used in packaging, textiles, biomedicine and other fields due to its excellent mechanical properties, biocompatibility and compostability. However, its actual environmental degradation efficiency is restricted by many factors such as temperature, humidity and microbial activity, which limits the full play of its sustainable development potential. Therefore, the development of an efficient and low-cost PLA waste recycling process has become the key to promoting a sustainable plastic economy.

[0003] The existing PLA recycling mainly includes mechanical recycling, pyrolysis and chemical depolymerization. Mechanical recycling, as the most common recycling method, has certain economic efficiency, but it often leads to a significant decline in material properties during the processing. Although pyrolysis can convert PLA into fuel oil or small molecule compounds, the pyrolysis temperature is usually 300–1000 °C, which requires a large amount of energy and may generate complex by-products, affecting the quality and purity of the products.

[0004] In contrast, chemical depolymerization shows significant advantages because it can efficiently degrade PLA into high-value-added small molecule monomers (such as lactate esters) under relatively mild conditions, as well as high-value-added chemicals for industrial products such as plastics, coatings, inks and dyes. The core of chemical depolymerization technology lies in the design of efficient catalysts. Traditional homogeneous acid-base catalysts (such as sulfuric acid, sodium hydroxide, etc.) and inorganic salt catalysts (such as chloride salts, iron salts, etc.) have high catalytic activity, but their corrosiveness and potential secondary pollution problems limit their sustainable application in industrialization.

[0005] In recent years, ionic liquids (such as [HSO3 - pmim][HSO4]) have attracted attention due to their adjustable acidity and alkalinity and high solubility in polymers. For example, Polym. Sci. 2014, 131 discloses a method for the degradation of polylactic acid using [HSO3 - pmim][HSO4] as a catalyst. Methyl lactate with a yield of 88.7% is obtained by reacting at 115 °C for 3.5 hours. However, due to the high preparation cost, high viscosity, poor mass transfer efficiency and complexity of the recycling process of [HSO3 - pmim][HSO4], its application in industry is greatly limited.

[0006] In contrast, heterogeneous catalysts can be recycled through simple solid-liquid separation, offering more sustainable advantages. For example, the sulfate-modified ZrO2 / SiO2 solid acid catalyst developed by Ye et al. can achieve a methyl lactate yield of 92.7% at 140 °C for 5 hours (Appl. Catalysis A, General 2023, 649, 118936). Therefore, developing an efficient, low-cost, and easily recyclable heterogeneous catalyst system is the key to improving the alcoholysis efficiency of PLA.

[0007] However, current heterogeneous catalysts still suffer from problems such as low catalyst efficiency, poor recyclability, harsh reaction conditions, and environmental pollution. Summary of the Invention

[0008] To solve the above problems, the present invention provides a method for catalyzing the degradation of polylactic acid with modified activated carbon. This method uses activated carbon modified step by step with nitric acid and sodium hydroxide as a catalyst to catalyze the degradation of polylactic acid, effectively promoting the polylactic acid degradation process, having high catalytic efficiency, easy catalyst recovery, and reducing the reaction energy consumption at the same time.

[0009] Therefore, the technical solution provided by the present invention is as follows: A method for catalyzing the degradation of polylactic acid with modified activated carbon, successively including the following steps: Step 1: Add activated carbon to water, heat and stir. After stirring ends, filter the liquid by suction, collect the filter cake and dry it to obtain activated AC; Step 2: Add all the AC prepared in Step 1 to an aqueous HNO3 solution, heat and stir, and condense and reflux for 3 - 15 h; after reflux ends, wash the product to neutrality and dry it overnight to obtain AC-H; Step 3: Transfer the dried AC-H to an aqueous NaOH solution, stir at room temperature, and then wash it to neutrality to obtain AC-H-Na; Step 4: Add polylactic acid, AC-H-Na, and anhydrous methanol to a reaction kettle, heat to 100 °C under a nitrogen atmosphere and react for 1 h to obtain methyl lactate; Wherein: The ratio of the activated carbon, HNO 3、 NaOH is 3.3 : 9.45 - 12.6 : 1.2.

[0010] The ratio of the polylactic acid, AC-H-Na, and anhydrous methanol is 200 mg : 10 mg : 25 mL.

[0011] Further, in the method for catalyzing the degradation of polylactic acid with modified activated carbon described above, the mass ratio of the activated carbon to water in Step 1 is 3.3 : 100.

[0012] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the heating and stirring in Step 1 is heating to 60 °C and stirring for 1 h.

[0013] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the drying in Step 1 is drying at 150 °C for 12 h.

[0014] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the molar concentration of the nitric acid solution in Step 1 is 6 - 8 mol / L.

[0015] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the heating in Step 2 is heating to 70 °C.

[0016] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the drying in Step 2 is overnight drying at 60 °C.

[0017] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the molar concentration of the sodium hydroxide solution in Step 3 is 1 mol / L, and the stirring time is 0.5 h.

[0018] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the reaction pressure in Step 4 is 1.0 MPa.

[0019] Further, in the method for catalytic degradation of polylactic acid by the above-mentioned modified activated carbon, the catalyst is one of AC - 6H6 - Na, AC - 8H3 - Na, AC - 8H6 - Na, AC - 8H9 - Na, AC - 8H12 - Na, AC - 8H15 - Na.

[0020] Compared with the prior art, the technical solution provided by the present invention has the following technical advantages: (1) The technical solution provided by the present invention uses nitric acid and sodium hydroxide to modify activated carbon step by step as a catalyst, avoiding the use of highly toxic reagents or precious metals. The reaction process does not require high temperature and high pressure. At 100 °C, PLA is almost completely degraded into methyl lactate in about 1 hour, and the yield can reach 100%, greatly reducing the reaction energy consumption and being significantly superior to traditional homogeneous catalysts and other heterogeneous catalysts; (2) The reaction by-products of the technical solution provided by the present invention are only water and trace amounts of unreacted methanol, without the emission of corrosive substances, reducing the risk of secondary pollution; and the catalyst can be recovered by simple filtration after the reaction, greatly reducing the catalyst loss and regeneration cost; compared with homogeneous catalysts and traditional solid acid catalysts, it significantly improves the process economy and is suitable for industrial continuous production; (3) The technical solution provided by the present invention uses activated carbon raw materials that are cheap and easily available. The preparation process only requires conventional acid treatment, alkali neutralization, and drying steps, without the need for complex equipment or harsh conditions, and is suitable for large-scale continuous production. The catalyst loading is low, with significant market competitiveness. Description of the Drawings

[0021] Figure 1 It is the X-ray diffraction characterization result diagram of AC and AC-8H9-Na provided in Example 4; Figure 2 It is the isothermal adsorption and desorption curve of AC and AC-8H9-Na provided in Example 4; Figure 3 It is the pore size distribution curve of AC and AC-8H9-Na provided in Example 4; Figure 4 It is the Raman spectroscopy characterization result diagram of AC and AC-8H9-Na provided in Example 4; Figure 5 It is the Fourier transform infrared spectroscopy characterization result diagram of AC and AC-8H9-Na provided in Example 4; Figure 6 It is the spectrogram of the C 1s fine spectrum of AC and AC-8H9-Na provided in Example 4 by XPS through peak fitting; Figure 7 It is the spectrogram of the Na 1s fine spectrum of AC and AC-8H9-Na provided in Example 4 by XPS through peak fitting; Figure 8 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 1; Figure 9 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 2; Figure 10 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 3; Figure 11 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 4; Figure 12 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 5; Figure 13 It is the result diagram of the gas chromatography analysis of the product after the reaction in Example 6; Figure 14 It is the result diagram of the gas chromatography analysis of the product after the reaction in Comparative Example 1; Figure 15 It is the result diagram of the gas chromatography analysis of the product after the reaction in Comparative Example 2; Figure 16 It is the result diagram of the gas chromatography analysis of the product after the reaction in Comparative Example 3; Figure 17 It is the result diagram of gas chromatography analysis of the product after the reaction of Comparative Example 4; Figure 18 It is the result diagram of gas chromatography analysis of the product after the reaction of Comparative Example 5; Figure 19 It is the result diagram of gas chromatography analysis of the product after the reaction of Comparative Example 6; Figure 20 It is the result diagram of gas chromatography analysis of the product after the reaction of Comparative Example 7; Figure 21 It is a comparison diagram of the catalyst types and the corresponding yields of methyl lactate in Examples 1-6 and Comparative Examples 1-7. Detailed implementation manners

[0022] In the description of the present invention, it should be noted that for those not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained by purchasing in the market.

[0023] Example 1

[0024] This example provides a method for catalytic degradation of polylactic acid by modified activated carbon, which successively includes the following steps: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After the stirring is completed, filter the liquid, collect the filter cake after filtration, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0025] Step 2: Add all the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 6 mol / L, and carry out condensation reflux at 600 rpm and heat to 70 °C for 6 h; after the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-6H6.

[0026] Step 3: Transfer the AC-6H6 prepared in Step 2 to 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm at room temperature for 0.5 h, and then wash it with deionized water until neutral, denoted as AC-6H6-Na.

[0027] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-6H6-Na prepared in Step 3 and 25 mL of anhydrous methanol to the reaction kettle, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. Analyze the reaction product by gas chromatography, and the chromatogram is referred to Figure 8 , and the yield of methyl lactate is 55.59%.

[0028] Step Five: After the reaction in Step Four is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid and catalyst AC-6H6-Na produced by the reaction) with a large amount of deionized water. After filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0029] Example 2

[0030] This example provides a method for the degradation of polylactic acid catalyzed by modified activated carbon, which successively includes the following steps: Step One: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0031] Step Two: Add all the AC prepared in Step One to 25 mL of HNO3 solution with a concentration of 8 mol / L, and carry out condensation reflux at 600 rpm and heat to 70 °C for 3 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until it is neutral, and dry it overnight at 60 °C, denoted as AC-8H3.

[0032] Step Three: Transfer the AC-8H3 prepared in Step Two to 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm at room temperature for 0.5 h, and then wash it with deionized water until it is neutral, denoted as AC-8H3-Na.

[0033] Step Four: Add 200 mg of polylactic acid particles, 10 mg of AC-8H3-Na prepared in Step Three and 25 mL of anhydrous methanol to the reaction kettle, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm and under the nitrogen condition of 1.0 MPa for 1 h to obtain methyl lactate. Analyze the reaction products by gas chromatography. The chromatogram is referred to Figure 9 , and the yield of methyl lactate is 79.17%.

[0034] Step Five: After the reaction in Step Four is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid and catalyst AC-8H3-Na produced by the reaction) with a large amount of deionized water. After filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0035] Example 3

[0036] This example provides a method for the degradation of polylactic acid catalyzed by modified activated carbon, which successively includes the following steps: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0037] Step 2: Add all of the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 8 mol / L, and carry out condensation reflux at 600 rpm and heat to 70 °C for 6 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-8H6.

[0038] Step 3: Transfer the AC-8H6 prepared in Step 2 to 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm at room temperature for 0.5 h, and then wash it with deionized water until neutral, denoted as AC-8H6-Na.

[0039] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-8H6-Na prepared in Step 3 and 25 mL of anhydrous methanol to the reaction kettle, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm, and 1.0 MPa nitrogen for 1 h to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is shown in Figure 10 , and the yield of methyl lactate is 82.85%.

[0040] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid and catalyst AC-8H3-Na generated by the reaction) by suction with a large amount of deionized water. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0041] Example 4

[0042] This example provides a method for the degradation of polylactic acid catalyzed by modified activated carbon, which successively includes the following steps: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0043] Step 2: Add all the AC prepared in Step 1 into 25 mL of HNO3 solution with a concentration of 8 mol / L, and carry out condensation reflux at 600 rpm and heated to 70 °C for 9 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-8H9.

[0044] Step 3: Transfer the AC-8H9 prepared in Step 2 into 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm and at room temperature for 0.5 h, and then wash it with deionized water until neutral, denoted as AC-8H9-Na.

[0045] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-8H9-Na prepared in Step 3 and 25 mL of anhydrous methanol into the reaction kettle, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm and under the nitrogen condition of 1.0 MPa for 1 h to obtain methyl lactate. Analyze the reaction product by gas chromatography, and refer to the chromatogram Figure 11 , and the yield of methyl lactate is 100.0%.

[0046] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted methanol liquid, methyl lactate liquid generated by the reaction and the catalyst AC-8H9-Na) with a large amount of deionized water. After filtration, collect the filter cake, and place the collected filter cake at 150 °C for drying for 12 h to recover the catalyst.

[0047] Example 5

[0048] This example provides a method for catalytic degradation of polylactic acid by modified activated carbon, which successively includes the following steps: Step 1: Take 3.3 g of activated carbon carrier, add it to 100 mL of water, heat it to 60 °C, stir at 60 °C for 1 h. After the stirring is completed, filter the liquid, collect the filter cake after filtration, and place the collected filter cake at 150 °C for drying for 12 h, denoted as AC.

[0049] Step 2: Add all the AC prepared in Step 1 into 25 mL of HNO3 solution with a concentration of 8 mol / L, and carry out condensation reflux at 600 rpm and heated to 70 °C for 12 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-8H12.

[0050] Step 3: Transfer the AC-8H12 prepared in Step 2 into 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm and at room temperature for 0.5 h, and then wash it with deionized water until neutral, denoted as AC-8H12-Na.

[0051] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-8H12-Na prepared in Step 3, and 25 mL of anhydrous methanol into the reaction kettle. Purge the reactor with nitrogen 5 times, and react for 1 h under the conditions of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is referred to Figure 12 , and the yield of methyl lactate is 97.77%.

[0052] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid and catalyst AC-8H12-Na generated by the reaction) with a large amount of deionized water. Collect the filter cake after filtration, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0053] Example 6

[0054] This example provides a method for the degradation of polylactic acid catalyzed by modified activated carbon, which successively includes the following steps: Step 1: Take 3.3 g of activated carbon support and add it to 100 mL of water. Heat to 60 °C and stir at 60 °C for 1 h. After stirring, filter the liquid, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0055] Step 2: Add all of the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 8 mol / L, and carry out condensation reflux at 600 rpm and heated to 70 °C for 15 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until it is neutral, and dry it overnight at 60 °C, denoted as AC-8H15.

[0056] Step 3: Transfer the AC-8H15 prepared in Step 2 to 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm and at room temperature for 0.5 h, and then wash it with deionized water until it is neutral, denoted as AC-8H15-Na.

[0057] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-8H15-Na prepared in Step 3, and 25 mL of anhydrous methanol into the reaction kettle. Purge the reactor with nitrogen 5 times, and react for 1 h under the conditions of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is referred to Figure 13 , and the yield of methyl lactate is 47.09%.

[0058] Step 5: After the reaction in Step 4 is completed, the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid and catalyst AC-8H15-Na produced by the reaction) are filtered by a large amount of deionized water. After filtration, the filter cake is collected and placed in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0059] To verify the performance of the catalyst provided by this application, the characterization results of catalyst AC and AC-8H9-Na in Example 4 are given below: The catalyst was characterized by X-ray diffraction (XRD), and the results are as Figure 1 shown. Through Figure 1 it can be seen that both AC and AC-8H9-Na before and after modification exhibit typical broad diffraction peaks of amorphous carbon, indicating that the graphite-like microcrystalline structure of activated carbon remains stable during the acid-base treatment process. Moreover, there is no residual NaOH and NaNO3 on AC and AC-8H9-Na, proving that the post-treatment process can effectively avoid the residue of inorganic salt by-products.

[0060] The pore structure characteristics of the material were analyzed by nitrogen adsorption-desorption experiments. The nitrogen adsorption–desorption isotherms and specific surface area changes of the samples are shown in Figure 2 , and through Figure 2 it can be seen that both AC and AC-8H9-Na exhibit typical Type-IV isotherm characteristics, indicating that both have mesoporous structures. Although AC-8H9-Na was modified with 8 mol / L nitric acid, it still maintained the mesoporous framework, indicating that this treatment did not destroy its overall mesoporous characteristics. However, its specific surface area decreased significantly from 827.4 m² / g to 531.4 m² / g. This obvious attenuation is attributed to the collapse of some microporous structures caused by the oxidation of nitric acid, thus reducing the surface area contributed by micropores. Figure 3 The change in the pore size distribution of the material is further shown. Through Figure 3 it can be seen that compared with the average pore size of the original AC, the average pore size of AC-8H9-Na increased from 3.7 nm to 4.0 nm, indicating that during the nitric acid oxidation treatment process, some micropores collapsed and pore channel reconstruction occurred, converting the original narrow pore channels into larger pores. This increase in the average pore size caused by the collapse of the pore structure is consistent with the trend of the decrease in specific surface area, jointly reflecting the deep regulation effect of nitric acid treatment on the pore structure of carbon materials.

[0061] The activated carbon before and after modification was characterized by Raman spectroscopy, and the results are as Figure 4 shown. It can be seen that the I D / I GThe ratio increased significantly compared to that of AC before modification, rising from 0.72 to 1.25. This change in the parameter intuitively reflects a substantial increase in the degree of disorder of the carbon skeleton induced by nitric acid oxidation treatment, further demonstrating that a large number of functional groups are generated on the carbon skeleton after nitric acid treatment.

[0062] FTIR spectra were used to further analyze the changes in the surface functional groups and elemental composition of carbon before and after modification. The spectra of AC and the AC-8H9-Na catalyst prepared by modification were compared, and the results are as Figure 5 shown. Among them, the absorption peak at 1362 cm -1 corresponds to HCO3 - , and the characteristic broad peak at 1597 cm -1 can be attributed to the stretching vibration mode of COO - . The absorption peaks at 2718 cm -1 and 2837 cm -1 can be attributed to the C–H stretching vibration in carboxylic acids. The broad absorption peak at 3432 cm -1 shows strong vibration characteristics of the O-H bond that may originate from the hydroxyl group in the carboxylic acid group. Compared with the original AC material, the intensity of the characteristic absorption peak at 1597 cm -1 of the treated AC-8H9-Na catalyst shows an increasing trend. This phenomenon indicates that high-concentration HNO3 oxidation and NaOH treatment can promote the formation of carboxylate groups on the surface of the activated carbon material.

[0063] XPS was used to further analyze the surface chemical state of the modified catalyst. By performing peak fitting on the C 1s fine spectrum, the results are as Figure 6 shown: The peaks at 284.8 eV, 285.4 eV, 286.6 eV, and 288.4 eV in the fine spectrum are attributed to sp2 bonds, sp3 bonds, C-O, and C=O, respectively. Compared with the original AC catalyst, the AC-8H9-Na catalyst shows a significantly enhanced peak intensity at the characteristic peak of the C=O bond, which confirms that nitric acid acidification treatment effectively increases the proportion of carboxyl functional groups on the surface of the activated carbon.

[0064] The Na 1s spectrum obtained synchronously, the results are as Figure 7 shown: A new characteristic peak appears at 1072.0 eV in the modified catalyst, and this peak position precisely corresponds to the chemical state of Na + in sodium carboxylate (-COONa). Combining the enhancement of the C=O bond in the C 1s spectrum ( Figure 6 ), it can be clearly seen that the post-treatment with NaOH promotes the deprotonation reaction of the surface carboxylic acid groups to form a sodium carboxylate structure.

[0065] Comparative Example 1 This comparative example serves as a blank control without adding any catalyst. The specific method for the degradation of polylactic acid is as follows: Add 200 mg of polylactic acid particles and 25 mL of anhydrous methanol into a reaction kettle, purge the reactor with nitrogen 5 times, and react for 1 h under the conditions of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction products by gas chromatography. The chromatogram is shown in Figure 14 , and the yield of methyl lactate is 5.47%.

[0066] Comparative Example 2 In this comparative example, AC was used as the catalyst. The specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0067] Step 2: Add 200 mg of polylactic acid particles, 10 mg of AC prepared in Step 1, and 25 mL of anhydrous methanol into a reaction kettle, purge the reactor with nitrogen 5 times, and react for 1 h under the conditions of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction products by gas chromatography. The chromatogram is shown in Figure 15 , and the yield of methyl lactate is 5.98%.

[0068] Step 3: After the reaction in Step 2 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC) with a large amount of deionized water by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0069] Comparative Example 3 In this comparative example, AC-Na was used as the catalyst. The specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0070] Step 2: Transfer all the AC prepared in Step 1 to 30 mL of a NaOH solution with a concentration of 1 mol / L, stir at 400 rpm and at room temperature for 0.5 h, and then wash with deionized water until neutral, denoted as AC-Na.

[0071] Step 3: Add 200 mg of polylactic acid particles, 10 mg of AC-Na prepared in Step 2, and 25 mL of anhydrous methanol into the reaction kettle. Purge the reactor with nitrogen 5 times. React for 1 h under the condition of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is referred to Figure 16 , and the yield of methyl lactate is 7.91%.

[0072] Step 4: After the reaction in Step 3 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC-Na) with a large amount of deionized water. Collect the filter cake after filtration. Place the collected filter cake in an oven at 150 °C and dry for 12 h to recover the catalyst.

[0073] Comparative Example 4 In this comparative example, AC-8H9 was used as the catalyst. The specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon support and add it to 100 mL of water. Heat to 60 °C and stir for 1 h while maintaining 60 °C. After stirring, filter the liquid. Collect the filter cake after filtration. Place the collected filter cake in an oven at 150 °C and dry for 12 h, denoted as AC.

[0074] Step 2: Add all of the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 8 mol / L. Stir at 600 rpm, heat to 70 °C, and carry out condensation reflux for 9 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until it is neutral, and dry it overnight at 60 °C, denoted as AC-8H9.

[0075] Step 3: Add 200 mg of polylactic acid particles, 10 mg of AC-8H9 prepared in Step 2, and 25 mL of anhydrous methanol into the reaction kettle. Purge the reactor with nitrogen 5 times. React for 1 h under the condition of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is referred to Figure 17 , and the yield of methyl lactate is 18.03%.

[0076] Step 4: After the reaction in Step 3 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC-8H9) with a large amount of deionized water. Collect the filter cake after filtration. Place the collected filter cake in an oven at 150 °C and dry for 12 h to recover the catalyst.

[0077] Comparative Example 5 In this comparative example, AC-1H6 was used as the catalyst. The specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0078] Step 2: Add all the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 1 mol / L, and carry out condensation reflux at 600 rpm and heated to 70 °C for 6 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-1H6.

[0079] Step 3: Transfer the AC-1H6 prepared in Step 2 to 30 mL of NaOH solution with a concentration of 1 mol / L, stir at 400 rpm and at room temperature for 0.5 h, and then wash it with deionized water until neutral, denoted as AC-1H6-Na.

[0080] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-1H6-Na and 25 mL of anhydrous methanol to the reaction kettle, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm and under a nitrogen condition of 1.0 MPa for 1 h to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is shown in Figure 18 , and the yield of methyl lactate is 16.95%.

[0081] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC-1H6-Na) with a large amount of deionized water by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h to recover the catalyst.

[0082] Comparative Example 6 In this comparative example, AC-2H6-Na was used as the catalyst, and the specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon support, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After stirring, filter the liquid by suction. After suction filtration, collect the filter cake, and place the collected filter cake in an oven at 150 °C for drying for 12 h, denoted as AC.

[0083] Step 2: Add all the AC prepared in Step 1 to 25 mL of HNO3 solution with a concentration of 2 mol / L, and carry out condensation reflux at 600 rpm and heated to 70 °C for 6 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-2H6.

[0084] Step 3: Transfer the AC-2H6 prepared in Step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm under normal temperature conditions for 0.5 h, and then wash with deionized water until neutral, denoted as AC-2H6-Na.

[0085] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-2H6-Na and 25 mL of anhydrous methanol into the reaction kettle, purge the reactor with nitrogen 5 times, and react under the conditions of nitrogen at 100 °C, 600 rpm and 1.0 MPa for 1 h to obtain methyl lactate. Analyze the reaction products by gas chromatography, and the chromatogram is referred to Figure 19 , and the yield of methyl lactate is 24.46%.

[0086] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC-2H6-Na) with a large amount of deionized water. After filtration, collect the filter cake, and place the collected filter cake at 150 °C for drying for 12 h to recover the catalyst.

[0087] Comparative Example 7 In this comparative example, AC-4H6-Na was used as the catalyst, and the specific method for the degradation of polylactic acid is as follows: Step 1: Take 3.3 g of activated carbon carrier, add it to 100 mL of water, heat to 60 °C, stir at 60 °C for 1 h. After the stirring is completed, filter the liquid, collect the filter cake after filtration, and place the collected filter cake at 150 °C for drying for 12 h, denoted as AC.

[0088] Step 2: Add all the AC prepared in Step 1 to 25 mL of 4 mol / L HNO3 solution, and carry out reflux condensation at 600 rpm and heated to 70 °C for 6 h. After the reflux is completed, wash the AC treated with HNO3 with deionized water until neutral, and dry it overnight at 60 °C, denoted as AC-4H6.

[0089] Step 3: Transfer the AC-4H6 prepared in Step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm under normal temperature conditions for 0.5 h, and then wash with deionized water until neutral, denoted as AC-4H6-Na.

[0090] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-4H6-Na prepared in Step 2, and 25 mL of anhydrous methanol into the reaction kettle. Purge the reactor with nitrogen 5 times. React for 1 h under the conditions of nitrogen at 100 °C, 600 rpm, and 1.0 MPa to obtain methyl lactate. Analyze the reaction product by gas chromatography. The chromatogram is referred to Figure 20 , and the yield of methyl lactate is 30.47%.

[0091] Step 5: After the reaction in Step 4 is completed, filter the substances in the reaction kettle (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC-4H6-Na) with a large amount of deionized water. After filtration, collect the filter cake. Place the collected filter cake at 150 °C and dry for 12 h to recover the catalyst.

[0092] For the analysis diagrams of the yields of methyl lactate in Examples 1 - 6 and Comparative Examples 1 - 7, refer to Figure 21 , by making a parallel comparison between different catalysts and the blank control, the yields of methyl lactate obtained on the untreated AC catalyst (Comparative Example 2) and the AC-Na catalyst treated only with NaOH (Comparative Example 3) are 5.98% and 7.91% respectively, which are similar to the result (5.47%) of the case without adding a catalyst (Comparative Example 1), indicating that the catalyst without carboxyl modification has almost no catalytic effect. At the same time, the yield of methyl lactate obtained on the AC-8H9 catalyst treated only with HNO3 (Comparative Example 4) is 18.03%, indicating that the carboxyl groups modified on the surface of activated carbon have certain catalytic activity for the alcoholysis of polylactic acid. The yield of methyl lactate of the AC-8H9-Na catalyst with carboxylate functional groups modified on the surface after sequential treatment with HNO3 and NaOH (Example 4) can reach 100.0% under the same conditions. This phenomenon may be attributed to the fact that sodium carboxylate (-COONa) loaded on the activated carbon serves as the acid-base active sites of the reaction, and the synergistic effect of the two reduces the activation energy barrier of ester bond hydrolysis.

[0093] The PLA alcoholysis performance of the AC-xH6-Na catalysts treated in HNO3 with different concentrations for 6 h was attempted (Examples 1, 3, Comparative Examples 5-7). For the AC catalyst without HNO3 treatment (Comparative Example 3), the methyl lactate yield was only 7.91%. As the concentration of the HNO3 solution gradually increased, the conversion of polylactic acid also gradually increased. This can be attributed to the fact that the increase in nitric acid concentration promoted the oxidation of the active surface and increased the carboxyl sites on the activated carbon surface. When the HNO3 concentration increased to 8 mol / L (Example 3), the methyl lactate yield could reach 82.85%. The catalyst was further optimized by changing the treatment time of activated carbon in 8 mol / L HNO3 (Examples 2-6). The catalyst with a treatment time of 9 h (AC-8H9-Na catalyst) showed the best performance. This is because appropriately increasing the treatment time can increase the functional groups on the activated carbon surface, while too long treatment time destroys the pore structure of the activated carbon and thus reduces the active sites of the catalyst.

[0094] In addition, compared with the methods for catalyzing the degradation of polylactic acid provided by the traditional homogeneous catalyst ionic liquid [HSO3 - pmim][HSO4] and the heterogeneous catalyst ZrO2 / SiO2 modified with sulfate for catalyzing the degradation of polylactic acid, the method for catalyzing the degradation of polylactic acid provided by the present application realizes the complete degradation of PLA into methyl lactate (ML) under mild conditions (100 °C, 1 h), and the yield reaches 100%, which is significantly better than that of the traditional ionic liquid [HSO3 - pmim][HSO4] reacting at 115 °C for 3.5 hours with a methyl lactate yield of 88.7%; and ZrO2 / SiO2 modified with sulfate reacting at 140 °C for 5 hours with a methyl lactate yield of 92.7%.

[0095] The present invention provides a method for catalyzing the degradation of polylactic acid. The catalyst used can be recovered by simple filtration after the reaction, greatly reducing the catalyst loss and regeneration cost; compared with the homogeneous catalyst ionic liquid that needs to be separated by distillation; the solid acid catalyst such as ZrO2 / SiO2 needs high-temperature regeneration, which significantly improves the process economy and is suitable for industrial continuous production.

[0096] The technical solution provided by the present invention uses nitric acid and sodium hydroxide to modify activated carbon step by step, avoiding the use of highly toxic reagents or precious metals and without high-temperature and high-pressure conditions. Experiments have proved that polylactic acid can be completely depolymerized into methyl lactate at 100 °C for 1 h, and the yield reaches 100%. The invention of this catalyst greatly reduces the energy consumption of the high-value recycling reaction of waste plastics, is significantly better than the traditional homogeneous catalyst, and has significant theoretical innovation value and potential industrial application prospects. In addition, the reaction by-products are only water and trace amounts of unreacted methanol, and there is no emission of corrosive substances, reducing the risk of secondary pollution.

Claims

1. A method for catalyzing the degradation of polylactic acid with modified activated carbon, characterized in that, The steps are as follows: Step 1: Add activated carbon to water, heat and stir, filter after stirring, collect the filter cake and dry it to obtain activated AC; Step 2: Add all the AC prepared in step 1 to HNO3 aqueous solution, heat and stir, and condense and reflux for 3-15h; after the reflux, wash the product to neutrality and dry overnight to obtain AC-H; Step 3: Transfer the AC-H prepared in step 2 into a NaOH aqueous solution, stir at room temperature, and then wash to neutrality to obtain AC-H-Na; Step 4: adding polylactic acid, AC-H-Na prepared in step 3 and anhydrous methanol into a reaction kettle, and reacting at 100° C. under a nitrogen environment to obtain methyl lactate; in: The activated carbon, HNO 3、 The mass ratio of NaOH is 3.3:9.45 - 12.6:1.2; The ratio of the polylactic acid, AC-H-Na and anhydrous methanol is 200 mg: 10 mg: 25 mL.

2. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The mass ratio of activated carbon to water in step 1 is 3.3:

100.

3. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The heating and stirring in step 1 is heating to 60° C. and stirring for 1 h.

4. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The drying in step 1 is performed at 150°C for 12 h.

5. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The molar concentration of the HNO3 aqueous solution in step 2 is 6-8 mol / L.

6. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The heating in step 2 is heating to 70°C.

7. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The drying in step 2 is carried out at 60° C. overnight.

8. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The molar concentration of the NaOH aqueous solution in step 3 is 1 mol / L, and the stirring time is 0.5 h.

9. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The reaction pressure of step 4 is 1.0 MPa and the reaction time is 1 h.

10. The method for catalyzing the degradation of polylactic acid with modified activated carbon according to claim 1, characterized in that, The catalyst is one of AC-6H6-Na, AC-8H3-Na, AC-8H6-Na, AC-8H9-Na, AC-8H12-Na, and AC-8H15-Na.

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