A method for catalytic degradation of polylactic acid by modified activated carbon

Abstract

The invention discloses a method for catalyzing the degradation of polylactic acid by modified activated carbon. The technical scheme comprises the following steps: step 1: adding activated carbon to water, heating and stirring, filtering after stirring, collecting solids and drying to obtain activated activated carbon; step 2: adding all the activated carbon to an HNO3 aqueous solution, heating and stirring, and condensing and refluxing for 3-15 hours; after the reflux is completed, washing the product to neutrality, and drying overnight to obtain activated carbon; step 3: transferring activated carbon to a NaOH aqueous solution, stirring at room temperature, and then washing to neutrality to obtain activated carbon, thereby obtaining activated carbon; and step 4: adding polylactic acid, activated carbon, and anhydrous methanol to a reactor, heating to a specified temperature under a nitrogen environment for reaction, thereby obtaining methyl lactate. The method uses activated carbon modified with nitric acid and sodium hydroxide in steps as a catalyst to catalyze the degradation of polylactic acid, has high catalytic efficiency, is easy to recover the catalyst, and reduces reaction energy consumption. The method belongs to the technical field of polymer degradation.

Description

Technical Field

[0001] The invention discloses a method for degrading polylactic acid, in particular to a method for degrading polylactic acid by catalyzing modified activated carbon, and belongs to the technical field of polymer degradation. Background Art

[0002] Polylactic acid (PLA), one of the most representative biodegradable polymers, has been widely used in packaging, textiles, and biomedicine due to its excellent mechanical properties, biocompatibility, and compostability. However, its actual environmental degradation efficiency is constrained by multiple factors, including temperature, humidity, and microbial activity, limiting its full potential for sustainable development. Therefore, developing efficient and low-cost recycling processes for PLA waste has become crucial for promoting a sustainable plastics economy.

[0003] Existing PLA recycling methods primarily include mechanical recycling, thermal cracking, and chemical depolymerization. Mechanical recycling, while the most common and economical method, often results in a significant degradation of material properties during processing. While thermal cracking can convert PLA into fuel oil or small molecule compounds, the pyrolysis temperature, typically between 300–1000°C, consumes significant energy and can generate complex byproducts, impacting product quality and purity.

[0004] In contrast, chemical depolymerization offers significant advantages, as it can efficiently degrade PLA into high-value-added small-molecule monomers (such as lactate) 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 and sodium hydroxide) and inorganic salt catalysts (such as chlorides and iron salts) have high catalytic activity, but their corrosiveness and potential for secondary pollution limit their sustainable application in industrial applications.

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

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

[0007] However, current heterogeneous catalysts still have 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 using modified activated carbon. The method uses nitric acid and sodium hydroxide to modify activated carbon in steps as a catalyst to catalyze the degradation of polylactic acid, effectively promoting the degradation process of polylactic acid, having high catalytic efficiency, and the catalyst is easy to recover, while reducing reaction energy consumption.

[0009] To this end, the technical solution provided by the present invention is as follows:

[0010] A method for catalyzing the degradation of polylactic acid using modified activated carbon comprises the following steps in sequence:

[0011] Step 1: Add activated carbon to water and heat and stir. After stirring, filter the liquid and collect the filter cake for drying to obtain activated AC.

[0012] Step 2: Add all the AC prepared in step 1 to an HNO3 aqueous solution, heat with stirring, and reflux under condensation for 3-15h; after the reflux is completed, wash the product until it is neutral and dry overnight to obtain AC-H;

[0013] Step 3: Transfer the dried AC-H to a NaOH aqueous solution, stir at room temperature, and then wash until neutral to obtain AC-H-Na;

[0014] Step 4: Add polylactic acid, AC-H-Na and anhydrous methanol into a reactor, raise the temperature to 100°C under a nitrogen environment and react for 1 hour to obtain methyl lactate;

[0015] in:

[0016] The activated carbon, HNO 3、 The ratio of NaOH is 3.3:9.45-12.6:1.2.

[0017] The ratio of the polylactic acid, AC-H-Na and anhydrous methanol is 200 mg:10 mg:25 mL.

[0018] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the mass ratio of the activated carbon to water in step 1 is 3.3:100.

[0019] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the heating and stirring in step 1 is heating to 60° C. and stirring for 1 hour.

[0020] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the drying in step 1 is performed at 150° C. for 12 h.

[0021] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the molar concentration of the nitric acid solution in step 1 is 6-8 mol / L.

[0022] Furthermore, in the above method for catalyzing the degradation of polylactic acid using modified activated carbon, the heating in step 2 is heating to 70°C.

[0023] Furthermore, in the above method for catalyzing the degradation of polylactic acid using modified activated carbon, the drying in step 2 is performed at 60° C. overnight.

[0024] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the molar concentration of the sodium hydroxide solution in step three is 1 mol / L, and the stirring time is 0.5 h.

[0025] Furthermore, in the above-mentioned method for catalyzing the degradation of polylactic acid using modified activated carbon, the reaction pressure in step four is 1.0 MPa.

[0026] Furthermore, in the above-mentioned method of catalyzing the degradation of polylactic acid using modified activated carbon, the catalyst is one of AC-6H6-Na, AC-8H3-Na, AC-8H6-Na, AC-8H9-Na, AC-8H12-Na, and AC-8H15-Na.

[0027] Compared with the prior art, the technical solution provided by the present invention has the following technical advantages:

[0028] (1) The present invention provides a technical solution that uses nitric acid and sodium hydroxide to modify activated carbon in steps 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 with a yield of up to 100%, greatly reducing the reaction energy consumption and significantly outperforming traditional homogeneous catalysts and other heterogeneous catalysts.

[0029] (2) The technical solution provided by the present invention produces only water and a trace amount of unreacted methanol as byproducts, with no discharge of corrosive substances, thus reducing the risk of secondary pollution. Moreover, the catalyst can be recovered by simple filtration after the reaction, significantly reducing catalyst loss and regeneration costs. Compared with homogeneous catalysts and traditional solid acid catalysts, the process economy is significantly improved, making it suitable for industrial continuous production.

[0030] (3) The technical solution provided by the present invention uses cheap and readily available activated carbon raw materials. The preparation process only requires conventional acid treatment, alkali neutralization, and drying steps, without the need for complex equipment or harsh conditions, making it suitable for large-scale continuous production. The catalyst loading is low, and it has significant market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the X-ray diffraction characterization result of AC and AC-8H9-Na provided in Example 4;

[0032] Figure 2 isothermal adsorption-desorption curves of AC and AC-8H9-Na provided in Example 4;

[0033] Figure 3 is the pore size distribution curve of AC and AC-8H9-Na provided in Example 4;

[0034] Figure 4 This is a graph showing the Raman spectroscopy characterization results of AC and AC-8H9-Na provided in Example 4;

[0035] Figure 5 is a graph showing the Fourier transform infrared spectroscopy characterization results of AC and AC-8H9-Na provided in Example 4;

[0036] Figure 6 The XPS spectra of AC and AC-8H9-Na provided in Example 4 were obtained by peak fitting of the C 1s fine spectrum;

[0037] Figure 7 The XPS spectra of AC and AC-8H9-Na provided in Example 4 were obtained by peak fitting of the Na 1s fine spectrum.

[0038] Figure 8 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 1;

[0039] Figure 9 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 2;

[0040] Figure 10 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 3;

[0041] Figure 11This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 4;

[0042] Figure 12 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 5;

[0043] Figure 13 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Example 6;

[0044] Figure 14 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 1;

[0045] Figure 15 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 2;

[0046] Figure 16 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 3;

[0047] Figure 17 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 4;

[0048] Figure 18 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 5;

[0049] Figure 19 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 6;

[0050] Figure 20 This is a graph showing the results of gas chromatography analysis of the product after the reaction in Comparative Example 7;

[0051] Figure 21 Comparison of catalyst types and corresponding methyl lactate yields in Examples 1-6 and Comparative Examples 1-7. DETAILED DESCRIPTION

[0052] In the description of the present invention, it should be noted that if specific conditions are not specified in the examples, the experiments were carried out according to conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used, if the manufacturer is not specified, are all conventional products that can be purchased commercially.

[0053] Example 1

[0054] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0055] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0056] Step 2: All the AC prepared in step 1 was added to 25 mL of 6 mol / L HNO3 solution and refluxed at 600 rpm and heated to 70°C for 6 h. After reflux, the HNO3-treated AC was washed with deionized water until neutral and dried at 60°C overnight. It was recorded as AC-6H6.

[0057] Step 3: Transfer the AC-6H6 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. The solution is recorded as AC-6H6-Na.

[0058] 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 into the reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 8 , the yield of methyl lactate was 55.59%.

[0059] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction and catalyst AC-6H6-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0060] Example 2

[0061] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0062] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0063] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution and reflux at 600 rpm and 70°C for 3 h. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-8H₃.

[0064] Step 3: Transfer the AC-8H3 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. The solution is recorded as AC-8H3-Na.

[0065] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-8H3-Na prepared in step 3, and 25 mL of anhydrous methanol into the reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. Analyze the reaction product by gas chromatography. See the chromatogram for details. Figure 9 , the yield of methyl lactate was 79.17%.

[0066] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction and catalyst AC-8H3-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0067] Example 3

[0068] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0069] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0070] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution and reflux at 600 rpm and 70°C for 6 h. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-8H₆.

[0071] Step 3: Transfer the AC-8H6 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. The solution is recorded as AC-8H6-Na.

[0072] 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 into the reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 10 , the yield of methyl lactate was 82.85%.

[0073] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction and catalyst AC-8H3-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0074] Example 4

[0075] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0076] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0077] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution and reflux at 600 rpm and 70°C for 9 h. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-8H9.

[0078] Step 3: Transfer the AC-8H9 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. The solution is recorded as AC-8H9-Na.

[0079] 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 reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Fig. Figure 11 , the yield of methyl lactate was 100.0%.

[0080] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-8H9-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0081] Example 5

[0082] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0083] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0084] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution and reflux at 600 rpm and 70°C for 12 h. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-8H₁₂.

[0085] Step 3: Transfer the AC-8H12 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. This solution is designated as AC-8H12-Na.

[0086] 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 reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 12 , the yield of methyl lactate was 97.77%.

[0087] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction and catalyst AC-8H12-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0088] Example 6

[0089] This embodiment provides a method for catalyzing the degradation of polylactic acid using modified activated carbon, which comprises the following steps in sequence:

[0090] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0091] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution and reflux at 600 rpm and 70°C for 15 h. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-8H15.

[0092] Step 3: Transfer the AC-8H15 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. This solution is designated as AC-8H15-Na.

[0093] 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 reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 13 , the yield of methyl lactate was 47.09%.

[0094] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction and catalyst AC-8H15-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0095] In order to verify the performance of the catalyst provided in this application, the characterization results of catalysts AC and AC-8H9-Na in Example 4 are given below:

[0096] The catalyst was characterized by X-ray diffraction (XRD). Figure 1 As shown. Figure 1 Both AC and AC-8H9-Na exhibit broad diffraction peaks typical of amorphous carbon before and after modification, indicating that the activated carbon's graphite-like microcrystalline structure remains stable during the acid-base treatment. Furthermore, no residual NaOH or NaNO3 remains on AC or AC-8H9-Na, demonstrating that the post-treatment process effectively avoids the accumulation of inorganic salt byproducts.

[0097] The pore structure characteristics of the material are analyzed through nitrogen adsorption-desorption experiments. The nitrogen adsorption-desorption isotherms and specific surface area changes of the samples are shown in Figure 2 ,pass Figure 2 As can be seen, both AC and AC-8H9-Na exhibit typical type IV isotherms, indicating that both possess a mesoporous structure. Despite treatment with 8 mol / L nitric acid, AC-8H9-Na maintains its mesoporous framework, indicating that this treatment does not destroy its overall mesoporous characteristics. However, its specific surface area decreases significantly from 827.4 m² / g to 531.4 m² / g. This significant decrease is attributed to the partial collapse of the microporous structure caused by nitric acid oxidation, thereby reducing the surface area contributed by the micropores. Figure 3 The pore size distribution of the material is further demonstrated by Figure 3 The average pore size of AC-8H9-Na increases from 3.7 nm to 4.0 nm compared to that of pristine AC. This indicates that during nitric acid oxidation, some micropores collapse and pore remodeling occurs, transforming the original narrow channels into larger pores. This increase in average pore size caused by pore structure collapse is consistent with the decreasing specific surface area trend, demonstrating the profound regulatory effect of nitric acid treatment on the pore structure of carbon materials.

[0098] Raman spectroscopy was used to characterize the activated carbon before and after modification. Figure 4 As shown. It can be seen that the I D / I G The ratio increased significantly from 0.72 to 1.25 relative to the AC before modification. This parameter change directly reflects the significant increase in carbon skeleton disorder induced by nitric acid oxidation, further proving that a large number of functional groups are generated in the carbon skeleton after nitric acid treatment.

[0099] FTIR spectra were used to further analyze the changes in carbon surface functional groups and elemental composition before and after modification. The spectra of AC and its modified AC-8H9-Na catalyst were compared. The results are as follows: Figure 5 As shown. Among them, 1362 cm -1 The absorption peak corresponds to HCO3 - , located at 1597 cm -1 The characteristic broad peak can be attributed to COO - Stretching vibration mode, 2718 cm -1 and 2837 cm -1 The absorption peak at 3432 cm can be attributed to the C–H stretching vibration of carboxylic acid. -1The broad absorption peak at 1597 cm-1 shows the strong vibration characteristics of the OH bond of the hydroxyl group in the carboxylic acid group. -1 The intensity of the characteristic absorption peak at shows an increasing trend, which indicates that high concentration HNO3 oxidation and NaOH treatment can promote the formation of carboxylate groups on the surface of activated carbon materials.

[0100] XPS was used to further analyze the surface chemical state of the modified catalyst. The results were as follows: Figure 6 As shown in the fine spectrum, the peaks at 284.8 eV, 285.4 eV, 286.6 eV, and 288.4 eV are attributed to sp2 bonds, sp3 bonds, CO, and C=O, respectively. Compared with the original AC catalyst, the AC-8H9-Na catalyst exhibits significantly enhanced peak intensity at the C=O bond characteristic peak, which confirms that the nitric acid treatment effectively increases the proportion of carboxyl functional groups on the activated carbon surface.

[0101] The Na 1s spectrum obtained simultaneously is as follows Figure 7 As shown: the modified catalyst shows a new characteristic peak at 1072.0 eV, which corresponds exactly to the Na + Chemical state. Combined with the C 1s spectrum ( Figure 6 ) can clearly indicate that the NaOH post-treatment induces the deprotonation reaction of the surface carboxylic acid groups to form sodium carboxylate structures.

[0102] Comparative Example 1

[0103] This comparative example is used as a blank control, without adding any catalyst, and the specific method of polylactic acid degradation is as follows:

[0104] 200 mg of polylactic acid particles and 25 mL of anhydrous methanol were added to the reactor. The reactor was purged with nitrogen 5 times. The reaction was carried out at 100 °C, 600 rpm, and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product was analyzed by gas chromatography. The chromatogram is shown in Fig. Figure 14 , the yield of methyl lactate was 5.47%.

[0105] Comparative Example 2

[0106] This comparative example uses AC as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0107] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0108] 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 reactor. Purge the reactor with nitrogen five times. React at 100 °C, 600 rpm, and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in FIG. Figure 15 , the yield of methyl lactate was 5.98%.

[0109] Step 3: After the reaction in step 2 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, catalyst AC) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150 ° C and dried for 12 hours to recover the catalyst.

[0110] Comparative Example 3

[0111] This comparative example uses AC-Na as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0112] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0113] Step 2: All the AC prepared in step 1 was transferred into 30 mL of 1 mol / L NaOH solution, stirred at 400 rpm and room temperature for 0.5 h, and then washed with deionized water until neutral, which was recorded as AC-Na.

[0114] 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 a reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in FIG. Figure 16 , the yield of methyl lactate was 7.91%.

[0115] Step 4: After the reaction in step 3 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0116] Comparative Example 4

[0117] This comparative example uses AC-8H9 as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0118] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0119] Step 2: Add all of the AC prepared in Step 1 to 25 mL of 8 mol / L HNO₃ solution, heat to 70°C at 600 rpm, and reflux under condensation for 9 hours. After reflux, wash the HNO₃-treated AC with deionized water until neutral and dry at 60°C overnight. This solution is designated AC-8H9.

[0120] 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 a reactor. Purge the reactor with nitrogen five times. React at 100 °C, 600 rpm, and 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in FIG. Figure 17 , the yield of methyl lactate was 18.03%.

[0121] Step 4: After the reaction in step 3 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-8H9) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0122] Comparative Example 5

[0123] This comparative example uses AC-1H6 as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0124] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0125] Step 2: Add all of the AC prepared in Step 1 to 25 mL of a 1 mol / L HNO3 solution and reflux at 600 rpm and 70°C for 6 h. After reflux, wash the HNO3-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-1H6.

[0126] Step 3: Transfer the AC-1H6 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. This solution is recorded as AC-1H6-Na.

[0127] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-1H6-Na and 25 mL of anhydrous methanol into the reactor, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 18 , the yield of methyl lactate was 16.95%.

[0128] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-1H6-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0129] Comparative Example 6

[0130] This comparative example uses AC-2H6-Na as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0131] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0132] Step 2: Add all of the AC prepared in Step 1 to 25 mL of a 2 mol / L HNO3 solution and reflux at 600 rpm and 70°C for 6 h. After reflux, wash the HNO3-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-2H6.

[0133] Step 3: Transfer the AC-2H6 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. The solution is recorded as AC-2H6-Na.

[0134] Step 4: Add 200 mg of polylactic acid particles, 10 mg of AC-2H6-Na and 25 mL of anhydrous methanol into the reactor, purge the reactor with nitrogen 5 times, and react at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 19 , the yield of methyl lactate was 24.46%.

[0135] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-2H6-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0136] Comparative Example 7

[0137] This comparative example uses AC-4H6-Na as a catalyst, and the specific method for degrading polylactic acid is as follows:

[0138] Step 1: Take 3.3 g of activated carbon carrier and add it to 100 mL of water, heat it to 60 °C, maintain 60 °C and stir for 1 hour. After stirring, filter the liquid and collect the filter cake. Place the collected filter cake at 150 °C and dry it for 12 hours. Record it as AC.

[0139] Step 2: Add all of the AC prepared in Step 1 to 25 mL of a 4 mol / L HNO3 solution and reflux at 600 rpm and 70°C for 6 h. After reflux, wash the HNO3-treated AC with deionized water until neutral and dry it at 60°C overnight. This solution is designated AC-4H6.

[0140] Step 3: Transfer the AC-4H6 prepared in step 2 to 30 mL of 1 mol / L NaOH solution, stir at 400 rpm and room temperature for 0.5 h, and then wash with deionized water until neutral. This solution is designated as AC-4H6-Na.

[0141] 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 reactor. Purge the reactor with nitrogen 5 times. React at 100 °C, 600 rpm, 1.0 MPa of nitrogen for 1 h to obtain methyl lactate. The reaction product is analyzed by gas chromatography. The chromatogram is shown in Figure 20 , the yield of methyl lactate was 30.47%.

[0142] Step 5: After the reaction in step 4 is completed, the substances in the reactor (unreacted polylactic acid and methanol liquid, methyl lactate liquid generated by the reaction, and catalyst AC-4H6-Na) are filtered with a large amount of deionized water, and the filter cake is collected after filtration. The collected filter cake is placed at 150°C and dried for 12 hours to recover the catalyst.

[0143] See the analysis chart of methyl lactate yield in Examples 1-6 and Comparative Examples 1-7. Figure 21 Comparing the different catalysts with a blank control, the yields of methyl lactate obtained with the untreated AC catalyst (Comparative Example 2) and the AC-Na catalyst treated only with NaOH (Comparative Example 3) were 5.98% and 7.91%, respectively, similar to the result of 5.47% obtained with no catalyst (Comparative Example 1), indicating that the catalysts without carboxyl modification had little catalytic effect. Meanwhile, the AC-8H9 catalyst treated only with HNO3 (Comparative Example 4) achieved an 18.03% yield of methyl lactate, demonstrating that the carboxyl groups modified on the activated carbon surface have some catalytic activity for the alcoholysis of polylactic acid. Furthermore, the AC-8H9-Na catalyst (Example 4), whose surface was modified with carboxylate functional groups after sequentially treating with HNO3 and NaOH (Example 4), achieved a 100.0% yield of methyl lactate under the same conditions. This phenomenon may be attributed to the presence of sodium carboxylate (-COONa) on the activated carbon, which acts as an acid-base active site for the reaction. The synergistic effect of the two reduces the activation barrier for ester bond hydrolysis.

[0144] The PLA alcoholysis performance of AC-xH6-Na catalysts treated in HNO3 at different concentrations for 6 h was investigated (Examples 1, 3, and Comparative Examples 5-7). For the AC catalyst without HNO3 treatment (Comparative Example 3), the methyl lactate yield was only 7.91%. With increasing HNO3 solution concentration, the conversion of polylactic acid also gradually increased. This can be attributed to the fact that increasing nitric acid concentration promotes oxidation of the active surface, increasing the number of carboxyl sites on the activated carbon surface. When the HNO3 concentration was increased to 8 mol / L (Example 3), the methyl lactate yield reached 82.85%. Further optimization of the catalysts was achieved by varying the treatment time of the activated carbon in 8 mol / L HNO3 (Examples 2-6). The catalyst treated for 9 h (AC-8H9-Na catalyst) exhibited the best performance. This is because appropriately increasing the treatment time increases the number of functional groups on the activated carbon surface, while excessive treatment time destroys the pore structure of the activated carbon, reducing the number of active sites.

[0145] In addition, the method for catalyzing the degradation of polylactic acid provided by the present application is different from the conventional homogeneous catalyst ionic liquid [HSO3 -Compared with the methods of polylactic acid degradation catalyzed by sulfate-modified ZrO2 / SiO2, the modified activated carbon-catalyzed polylactic acid degradation method provided in this application can achieve complete degradation of PLA to methyl lactate (ML) under mild conditions (100 ° C, 1 h) with a yield of 100%, which is significantly better than the traditional ionic liquid [HSO3 - pmim][HSO4] reacted at 115℃ for 3.5 hours, and the yield of methyl lactate was 88.7%; sulfate-modified ZrO2 / SiO2 reacted at 140℃ for 5 hours, and the yield of methyl lactate was 92.7%.

[0146] 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 catalyst loss and regeneration costs. Compared with homogeneous catalysts, ionic liquids require distillation separation; solid acid catalysts such as ZrO2 / SiO2 require high-temperature regeneration, which significantly improves the process economy and is suitable for industrial continuous production.

[0147] The present invention provides a technical solution that uses nitric acid and sodium hydroxide to modify activated carbon in steps, avoiding the use of highly toxic reagents or precious metals and eliminating the need for high temperature and high pressure conditions. Experiments have shown that polylactic acid can be completely depolymerized into methyl lactate at 100°C in 1 hour, with a yield of 100%. The invention of this catalyst greatly reduces the energy consumption of high-value recycling reactions for waste plastics, significantly outperforming traditional homogeneous catalysts and possessing significant theoretical innovation value and potential industrial application prospects. In addition, the reaction byproducts are only water and trace amounts of unreacted methanol, with no corrosive substances emitted, reducing the risk of secondary pollution.

Claims

1. A method for catalyzing the degradation of polylactic acid using modified activated carbon, characterized in that: The method includes the following steps in sequence: 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 an HNO3 aqueous solution, heat with stirring, and reflux under condensation for 3-15 hours; after the reflux is completed, wash the product until it is neutral 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 until neutral to obtain AC-H-Na; Step 4: Add polylactic acid, AC-H-Na prepared in step 3 and anhydrous methanol into a reactor and react at 100° C. under a nitrogen environment to obtain methyl lactate; in: The mass ratio of the activated carbon, HNO3, and NaOH is 3.3:9.45-12.6:1.2; The ratio of polylactic acid, AC-H-Na and anhydrous methanol is 200 mg: 10 mg: 25 mL; The molar concentration of the HNO3 aqueous solution described in step 2 is 6-8 mol / L.

2. The method for catalyzing the degradation of polylactic acid using modified activated carbon according to claim 1, wherein: 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 using modified activated carbon according to claim 1, wherein: The heating and stirring in step 1 is heating to 60° C. and stirring for 1 hour.

4. The method for catalyzing the degradation of polylactic acid using modified activated carbon according to claim 1, wherein: The drying step is carried out at 150° C. for 12 h.

5. The method for catalyzing the degradation of polylactic acid using modified activated carbon according to claim 1, wherein: The heating in step 2 is heating to 70°C.

6. The method for catalyzing the degradation of polylactic acid using modified activated carbon according to claim 1, wherein: The drying in step 2 is carried out at 60° C. overnight.

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

8. The method for catalyzing the degradation of polylactic acid using modified activated carbon according to claim 1, wherein: The reaction pressure of step 4 is 1.0 MPa and the reaction time is 1 h.

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