Method for preparing polylactic acid PLA from straw lignocellulose

Abstract

The invention relates to the technical field of polylactic acid preparation, in particular to a method for preparing polylactic acid PLA by utilizing straw lignocellulose, which comprises the following steps: crushing straws to 0.5-2mm, adding 3-8% of composite pretreating agent and 0.1-0.5% of nano TiO2, reacting at 80-100 DEG C for 2-4 hours, and filtering; adding 1-3% of alkaline protease for enzymolysis, then adding 2-5% of sodium hydroxide and 0.05-0.2% of chitosan for heat preservation, washing and drying to obtain lignocellulose; mixing and saccharifying with 0.5-2% of compound saccharifying enzyme and 0.02-0.1% of [Emim] Ac, and filtering to obtain a glucose solution; adding a fermentation aid and graphene quantum dots, and inoculating lactic acid bacteria for anaerobic fermentation; adding activated carbon for adsorption, filtering, concentrating, cooling, crystallizing, washing and drying to obtain a pure lactic acid product; carrying out vacuum oligomerization on lactic acid at 130-150 DEG C, adding a catalyst, cracking at 180-200 DEG C, and purifying to obtain lactide; adding a catalyst, montmorillonoid and an antioxidant 1010 into the lactide, and polymerizing to obtain the PLA. According to the method, the straw lignocellulose extraction rate and enzymolysis efficiency are improved, energy consumption and wastewater are reduced, PLA purity, mechanical properties and degradability are excellent, cost is reduced, and the method meets the requirements of environmental protection and resource circulation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polylactic acid preparation, in particular to a method for preparing polylactic acid (PLA) from straw lignocellulose. BACKGROUND

[0002] As a typical biobased degradable polymer material, polylactic acid (PLA) has a wide application prospect in the fields of packaging, medical treatment, agriculture, etc. due to its good biocompatibility, mechanical processability and complete degradation characteristics, and is one of the core materials to replace traditional petroleum-based plastics. At present, the industrial production of PLA mainly uses starch extracted from food crops such as corn and sugarcane as raw material, and is prepared through the processes of saccharification, fermentation and polymerization. However, this technical path has two major problems: first, it depends on food resources, is significantly affected by the global food supply and demand relationship and price fluctuations, and is easy to cause food safety controversy in the process of "food to plastic"; second, the cost of raw materials accounts for 60%-70% of the production cost of PLA, and the high cost of raw materials limits the large-scale popularization and application of PLA.

[0003] In order to solve the problems of raw material dependence and cost, the industry gradually turns its attention to the resource utilization of agricultural waste. Straw, as the largest agricultural waste, its main component lignocellulose can be converted into glucose through degradation, and then used for PLA preparation, becoming a potential non-food raw material. However, the structural characteristics of straw lignocellulose pose a serious challenge to its efficient conversion: the cell wall of straw is formed by the rigid structure of cellulose, hemicellulose and lignin, which are tightly cross-linked. The hydrophobic properties and spatial barrier effect of lignin can significantly inhibit the efficiency of subsequent enzymatic hydrolysis and saccharification. At the same time, the impurities such as ash, pectin and protein contained in the straw not only reduce the activity of the enzyme, but also inhibit the growth of microorganisms in the fermentation stage, resulting in a decrease in lactic acid yield.

[0004] In the existing technical solutions for preparing PLA from straw, the pretreatment stage mostly uses single acid hydrolysis, alkali hydrolysis or high-temperature cooking process. Single acid hydrolysis can destroy the structure of lignin, but it easily leads to excessive degradation of cellulose to generate inhibitory byproducts such as furfural, and the equipment is severely corroded. Single alkali hydrolysis can remove part of hemicellulose and lignin, but the reaction conditions are severe, the energy consumption is high, and a large amount of alkaline wastewater is generated, which has high environmental protection treatment cost. High-temperature cooking needs to be carried out under high temperature and high pressure conditions of 180-220℃, which not only has huge energy consumption, but also destroys the crystal structure of cellulose, reducing the efficiency of subsequent saccharification. In addition, in the enzymatic saccharification stage, the traditional scheme mostly uses single cellulase, which has insufficient degradation ability for hemicellulose, resulting in a utilization rate of straw carbohydrate of only 40%-50%, and a large amount of hemicellulose resources is wasted. In the fermentation stage, there is a lack of targeted fermentation aids, and the activity of lactic acid bacteria is easily inhibited by impurities, resulting in a lactic acid yield of less than 70g / L, and the purity of the product is insufficient, which requires multiple rectifications in the subsequent purification, further increasing the energy consumption and cost.

[0005] In the PLA polymerization link, the prior art mostly uses single stannous octoate as a catalyst, which can realize lactic acid polymerization, but the catalytic efficiency is low, and it needs to react at high temperature of 180-200℃ and high vacuum condition for 12-16 hours, which not only has high energy consumption, but also easily leads to PLA molecular chain rupture, affecting the molecular weight and mechanical properties of the product. At the same time, there is a lack of effective dispersion and stabilization mechanism in the polymerization process, and PLA is prone to local overheating carbonization, further reducing the product quality. In addition, the whole preparation process lacks a systematic impurity control scheme, and the ash in the straw, the residual acid and alkali substances in the pretreatment, and the metabolic impurities in the fermentation stage will accumulate in each link, finally affecting the purity and performance of PLA, making it difficult to meet the needs of high-end fields.

[0006] With the promotion of the "double carbon" target and the upgrading of environmental protection requirements, higher requirements are put forward for the diversification of PLA raw materials, cost reduction and performance improvement. Due to the low pretreatment efficiency, insufficient resource utilization, high energy consumption, and unstable product quality of traditional technology, it has been difficult to meet the needs of industry development. Therefore, developing a PLA preparation technology that can efficiently break down the structure of straw lignocellulose, improve resource utilization, reduce energy consumption and cost, and ensure product quality has become a key to promoting the utilization of straw resources and the sustainable development of the PLA industry. SUMMARY

[0007] (I) Technical problems solved

[0008] In view of the shortcomings of the prior art, the present application provides a method for preparing polylactic acid (PLA) from straw lignocellulose.

[0009] (II) Technical solutions

[0010] A method for preparing polylactic acid (PLA) from straw lignocellulose, comprising the following steps:

[0011] S1, straw pretreatment: crushing the straw to a particle size of 0.5-2mm, adding a composite pretreatment agent with a mass fraction of 3-8% and nano-TiO2 with a mass fraction of 0.1-0.5%, stirring and reacting at 80-100℃ for 2-4 hours, and filtering to obtain pretreated straw;

[0012] S2, lignocellulose extraction: adding an alkaline protease solution with a mass fraction of 1-3% to the pretreated straw, enzymatic hydrolysis at 45-55℃ for 1.5-3 hours, then adding sodium hydroxide solution and chitosan, and incubating at 60-75℃ for 2-3 hours, washing to neutral after filtration, and drying to obtain lignocellulose;

[0013] S3, cellulose saccharification: mixing lignocellulose with 0.5-2% of complex saccharifying enzyme and 0.02-0.1% of ionic liquid [Emim]Ac by mass fraction, controlling solid-liquid ratio 1:8-12, saccharifying at 50-60℃ and pH 4.5-5.5 for 3-6 hours, and filtering to obtain glucose solution;

[0014] S4, lactic acid fermentation: adding 0.1-0.5% of fermentation aid and 0.01-0.05% of graphene quantum dots by mass fraction to the glucose solution, inoculating 5-10% of lactic acid bacteria, and anaerobically fermenting at 35-42℃ and pH 6.0-7.0 for 48-72 hours to obtain lactic acid solution;

[0015] S5, lactic acid purification: adding 5-10% of activated carbon by mass fraction to the lactic acid solution, stirring and adsorbing at 60-80℃ for 1-2 hours, filtering to obtain preliminarily purified lactic acid solution; concentrating the solution at 40-50℃ and vacuum degree -0.08 to -0.09 MPa to lactic acid mass fraction 50-60%; controlling cooling rate 5-8℃ / h, slowly cooling from room temperature to 5-10℃, and standing for 4-6 hours to make lactic acid crystallize; centrifugally separating the crystals from the mother liquor, washing the crystals with 5-10℃ deionized water for 2-3 times, each time using water amount 10-15% of the mass of the crystals; and finally drying at 40-50℃ and vacuum degree -0.08 to -0.09 MPa for 2-3 hours to obtain lactic acid pure product with purity ≥99.8%;

[0016] S6, lactide preparation: adding lactic acid pure product into a reaction kettle, vacuum oligomerizing at 130-150℃ and vacuum degree -0.095 to -0.1 MPa for 3-5 hours; subsequently adding 0.1-0.3% of complex catalyst by mass of lactic acid pure product, increasing temperature to 180-200℃ and maintaining the above vacuum degree, cracking and cyclization for 6-8 hours, and condensing and collecting crude lactide; recrystallizing the crude lactide with 95% ethanol by mass for 2-3 times, and drying at 80-90℃ and vacuum degree -0.09 MPa for 4-6 hours to obtain lactide fine product with purity ≥99.5%;

[0017] S7, polylactic acid polymerization: mixing lactide fine product with 0.08-0.25% of complex catalyst, 0.03-0.1% of montmorillonite and 0.01-0.03% of antioxidant 1010 by mass fraction, preheating at 120-130℃ and vacuum degree -0.09 to -0.1 MPa for 1-2 hours, subsequently increasing temperature to 160-170℃ and maintaining the above vacuum degree, and polymerizing at stirring rate 20-40 rpm for 8-12 hours to obtain polylactic acid PLA.

[0018] Preferably, the process also includes straw activation: placing the straw in a microwave reactor and pretreating it for 5-10 minutes at a power of 300-500W and a frequency of 2450MHz to increase the porosity of the straw fibers by 20-30% before crushing.

[0019] Preferably, the process also includes sugar solution purification: filtering the glucose solution through a 0.1-0.2μm ceramic membrane, controlling the operating pressure at 0.1-0.2MPa, retaining protein impurities with a molecular weight of 500-1000Da, and collecting the permeate as fermentation feedstock.

[0020] Preferably, the nano-TiO2 in S1 is anatase type, with a particle size of 10-30 nm and a specific surface area of ​​50-80 m². 2 / g, modified with silane coupling agent KH550 before use, at a temperature of 60-80℃ for 1-2 hours; the composite pretreatment agent is a mixture of citric acid and oxalic acid in a mass ratio of 2:1.

[0021] Preferably, the degree of deacetylation of chitosan in S2 is not less than 90%, the viscosity is 100-200 mPa·s, the amount added is 0.5-1.5% of the mass of the pretreated straw, and the stirring speed is controlled at 80-120 rpm during the heat preservation process.

[0022] Preferably, the purity of [Emim]Ac in S3 is not less than 98%, the water content is not more than 0.5%, and the mass ratio of [Emim]Ac to lignocellulose is 1:5-10. During the saccharification process, the mixture is stirred once every 1 hour for 5 minutes each time. The compound saccharifying enzyme is composed of cellulase and hemicellulase in a mass ratio of 3:1.

[0023] Preferably, the graphene quantum dots in S4 have a particle size of 2-10 nm, a surface carboxyl content of not less than 5 wt%, an excitation wavelength of 360-400 nm, and a fluorescence quantum yield of not less than 30%. Nitrogen gas needs to be introduced during fermentation to maintain an anaerobic environment, with a nitrogen flow rate of 0.5-1 L / min. The fermentation aid is a mixture of yeast extract and ammonium sulfate in a mass ratio of 1:2.

[0024] Preferably, the crystallization step in S5 adopts a two-stage cooling procedure: first, the solution is cooled from room temperature to 20-25°C at a rate of 3-5°C / h; when the supersaturation of the solution reaches 1.2-1.5 times, it is then cooled to 5-8°C at a rate of 8-10°C / h; and 0.5-1% by mass of lactic acid seed crystals are added to the concentrated lactic acid solution in advance, and the seed crystals are added and stirred evenly 30 minutes before the first cooling.

[0025] Preferably, the montmorillonite in S7 is sodium-based, with an interlayer spacing of 1.5-2.0 nm and a cation exchange capacity of 80-120 mmol / 100g. It is organically modified by hexadecyltrimethylammonium bromide at a temperature of 80-90℃ for 2-4 hours.

[0026] Preferably, the S7 polymerization stage adopts a gradient heating mode: 160℃ for 2 hours, 170℃ for 3 hours, and 180℃ for 1-5 hours. The vacuum degree of each stage is increased by 0.005MPa compared with the previous stage, and the stirring rate is controlled at 20-40rpm. The composite catalyst is composed of stannous octoate and zinc oxide in a mass ratio of 4:1.

[0027] (III) Beneficial Technical Effects

[0028] Compared with existing technologies, the beneficial effects of this invention are:

[0029] 1. The citric acid and oxalic acid in the composite pretreatment agent can gently disrupt the cross-linking structure of lignin and hemicellulose, while nano-TiO2 can further degrade lignin through photocatalysis, while reducing excessive degradation of cellulose. Compared with traditional single acid or alkali hydrolysis processes, the pretreatment temperature is lower, the reaction time is shorter, and no large amount of harmful wastewater is generated, significantly improving environmental friendliness. The subsequent addition of chitosan can adsorb ash and impurities in the straw, reducing inhibition of subsequent enzymatic hydrolysis and fermentation, greatly improving the extraction rate and purity of lignocellulose, laying the foundation for efficient subsequent conversion.

[0030] 2. The cellulase and hemicellulase in the compound saccharifying enzyme can degrade cellulose and hemicellulose respectively, while the ionic liquid can disrupt the cellulose crystal structure, improving enzyme accessibility and increasing the utilization rate of straw carbohydrates. Compared with traditional single enzymatic hydrolysis methods, resource waste is significantly reduced. The graphene quantum dots added during the fermentation stage can activate lactic acid bacteria activity, providing nutrients to the bacteria in conjunction with fermentation aids, effectively resisting impurity inhibition, and significantly improving lactic acid yield and purity. Subsequent purification steps are simplified, reducing energy consumption and costs. Simultaneously, the newly added sugar solution purification step can further remove protein impurities, ensuring the purity of fermentation raw materials and avoiding the impact of impurities on lactic acid bacteria.

[0031] 3. Activated carbon effectively adsorbs pigments and organic impurities, while zeolite adsorbs moisture and small molecule impurities. The polymerization stage utilizes a composite catalyst and montmorillonite working synergistically. The stannous octoate and zinc oxide in the composite catalyst enhance catalytic efficiency and shorten polymerization time, while montmorillonite disperses the polymerization system, preventing localized overheating and improving the mechanical properties and thermal stability of PLA. Compared to traditional single-catalyst solutions, this method results in lower polymerization temperature, shorter reaction time, significantly reduced energy consumption, and a more uniform PLA molecular weight distribution, leading to improved product quality. Furthermore, the gradient heating mode in the polymerization stage further optimizes the molecular chain growth process, preventing chain breakage and ensuring the molecular weight and mechanical properties of PLA. Attached Figure Description

[0032] Figure 1This is a flowchart of a method for preparing polylactic acid (PLA) using straw lignocellulose proposed in this invention;

[0033] Figure 2 This is a line graph comparing the lignocellulose extraction rate and glucose conversion rate of the examples and comparative examples;

[0034] Figure 3 This is a bar chart comparing the number-average molecular weight and tensile strength of PLA in the examples and comparative examples;

[0035] Figure 4 This is a radar comparison chart created by standardizing the dimensions of the performance test results of the examples and comparative examples. Detailed Implementation

[0036] according to Figures 1 to 4 The specific embodiments of the present invention are as follows:

[0037] Raw material and equipment preparation

[0038] Raw material specifications

[0039] Straw: Corn straw, wheat straw, and rice straw are selected and naturally dried to a moisture content of 10-15%. Before crushing, it needs to be treated to remove impurities such as mud, sand, and metal.

[0040] Chemical reagents: Citric acid purity not less than 99%, oxalic acid purity not less than 99%, nano-TiO2 anatase type with a particle size of 10-30 nm, silane coupling agent KH550 purity not less than 97%; alkaline protease activity 10000-15000 U / g, chitosan deacetylation degree not less than 90%, sodium hydroxide purity not less than 96%; cellulase activity 5000 U / g, hemicellulase activity 3000 U / g, ionic liquid [Emim]Ac purity not less than 98%; yeast extract is biological... Reagent grade; ammonium sulfate is analytical grade; graphene quantum dots have a particle size of 2-10 nm; lactic acid bacteria are a mixed strain of Lactobacillus delbrueckii and Lactobacillus plantarum; activated carbon has a particle size of 100-200 mesh; ethanol has a purity of 95%; 4A molecular sieve has a particle size of 5-10 μm; stannous octoate has a purity of not less than 99%; zinc oxide has a purity of not less than 99.5%; sodium montmorillonite has a cation exchange capacity of 80-120 mmol / 100g; cetyltrimethylammonium bromide has a purity of not less than 99%; antioxidant 1010 is industrial grade with a purity ≥99%.

[0041] Equipment list

[0042] Pretreatment equipment: microwave reactor with power 0-1000W and frequency 2450MHz, high-speed pulverizer with adjustable particle size 0.1-5mm, constant temperature stirred reactor with temperature 0-200℃ and stirring speed 0-500rpm, and vacuum filter with filtration accuracy of 0.1μm.

[0043] Enzymatic hydrolysis and fermentation equipment: constant temperature water bath shaker with temperature 0-100℃ and speed 0-300rpm, anaerobic fermenter with volume 5-50L and equipped with pH and temperature control system, ceramic membrane filtration system with membrane pore size 0.1-0.2μm and operating pressure 0-0.5MPa.

[0044] Purification and polymerization equipment: vacuum concentrator with vacuum degree of -0.1 to 0 MPa and temperature of 0-100℃, programmable temperature crystallizer with temperature of -5 to 60℃ (temperature control accuracy ±0.5℃, stirring speed 0-200 rpm), horizontal screw centrifuge with separation factor of 1000-3000g, low temperature washing water system with temperature of 5-10℃ (including metering pump), twin-screw extruder with temperature of 0-300℃ and screw speed of 0-500 rpm, vacuum polymerization reactor with gradient heating and stirring function, lactide condensation and collection device, crystallizer (with temperature control and stirring).

[0045] Testing equipment: High performance liquid chromatography (HPLC) is used to detect lactic acid purity, gel permeation chromatography (GPC) is used to detect PLA molecular weight, universal testing machine is used to detect mechanical properties, and thermogravimetric analyzer (TGA) is used to detect thermal stability.

[0046] Example 1

[0047] Straw activation: 1000g of corn straw was placed in a microwave reactor, and the power was set to 400W and the frequency to 2450MHz for pretreatment for 8 minutes. After cooling, the straw fiber porosity was measured to be 25% higher.

[0048] Straw pretreatment: Activated straw was crushed to a particle size of 1 mm, and a 5% (w / w) composite pretreatment agent was added. The composite pretreatment agent was composed of 50 g of citric acid and 25 g of oxalic acid. Simultaneously, 0.3% (w / w) of nano-TiO2 was added, which had been modified with KH550 at 70℃ for 1.5 hours. 2000 mL of deionized water was added, and the mixture was reacted at 90℃ and a stirring speed of 150 rpm for 3 hours. The pretreated straw was then obtained by vacuum filtration.

[0049] Lignocellulose extraction: A 2% (w / w) alkaline protease solution (12000 U / g activity) was added to pretreated straw, at a concentration of 15% of the straw mass. Enzymatic hydrolysis was carried out at 50℃ and 100 rpm for 2 hours. Then, a 3% (w / w) sodium hydroxide solution and 0.1% (w / w) chitosan (150 mPa·s viscosity) were added, at a concentration of 1% of the straw mass. The mixture was kept at 70℃ and 100 rpm for 2.5 hours. After filtration, the mixture was washed with deionized water until pH 7.0 and dried at 80℃ to obtain 520 g of lignocellulose.

[0050] Cellulose saccharification: Lignocellulose was mixed with a 1% (w / w) compound saccharifying enzyme, which consisted of 30g of cellulase and 10g of hemicellulase. Simultaneously, 0.05% (w / w) of the ionic liquid [Emim]Ac was added, with a mass ratio of 1:8 to lignocellulose. The solid-liquid ratio was controlled at 1:10, and deionized water was added to adjust the pH to 5.0. Saccharification was carried out at 55℃ for 4 hours, with stirring for 5 minutes every hour during the process. The solution was then filtered to obtain a glucose solution.

[0051] Glucose solution purification: The glucose solution is filtered through a 0.15μm ceramic membrane at an operating pressure of 0.15MPa to remove protein impurities with a molecular weight of 800Da, and the permeate is collected.

[0052] Lactic acid fermentation: A fermentation aid of 0.3% (w / w) was added to the purified glucose solution. The fermentation aid consisted of a mixture of 10g yeast extract and 20g ammonium sulfate. Simultaneously, 0.03% (w / w) of graphene quantum dots with a surface carboxyl group content of 6wt% was added. 8% lactic acid bacteria (a 1:1 mixture of *Lactobacillus delbrueckii* and *Lactobacillus plantarum*) were inoculated. Anaerobic fermentation was carried out at 38℃ and pH 6.5 for 60 hours, with nitrogen gas introduced during the fermentation process at a flow rate of 0.8 L / min. The resulting lactic acid solution was obtained by filtration.

[0053] Lactic acid purification: 8% activated carbon was added to the lactic acid solution, and adsorption was carried out at 70℃ and a stirring speed of 120 rpm for 1.5 hours. The solution was filtered to obtain a preliminarily purified lactic acid solution. This solution was concentrated to a lactic acid mass fraction of 55% at 405℃ and a vacuum of -0.08 MPa. The cooling rate was controlled at 68℃ / h, and the solution was slowly cooled from room temperature to 6℃ and kept at this temperature for 5 hours to allow lactic acid crystals to precipitate. The crystals were separated from the mother liquor by centrifugation, and the crystals were washed three times with deionized water at 5℃, with each wash using 12% of the crystal mass of water. Finally, the solution was dried at 40℃ and a vacuum of -0.08 MPa for 3 hours to obtain pure lactic acid with a purity ≥99.8%.

[0054] Preparation of lactide: Pure lactic acid was added to an oligomerization reactor and oligomerized under vacuum at 130℃ and -0.095MPa for 3 hours. 0.1% (by weight of pure lactic acid) of a composite catalyst (8g stannous octoate + 2g zinc oxide) was added, and the temperature was raised to 180℃ while maintaining the same vacuum. The pyrolysis and cyclization reaction was carried out for 6 hours, and the crude lactide was collected by condensation. 95% ethanol was added to the crude lactide at a solid-liquid ratio of 1:5, and the mixture was kept at 20℃ for 1 hour before filtration. The mixture was recrystallized twice and then dried at 80℃ and -0.09MPa for 4 hours to obtain a high-purity lactide product with 99.6% purity.

[0055] Polylactic acid polymerization: High-quality lactide is mixed with 0.08% by mass of a composite catalyst (prepared in the same way as lactide), 0.06% by mass of organic modified montmorillonite, and 0.01% by mass of antioxidant 1010, and added to a polymerization reactor; preheated at 120°C and vacuum degree -0.09MPa for 1 hour, then heated to 160°C and maintained at vacuum degree, and polymerized at a stirring rate of 30rpm for 8 hours to obtain polylactic acid PLA.

[0056] Example 2

[0057] Straw activation: 1000g of wheat straw was placed in a microwave reactor, and the power was set to 350W and the frequency to 2450MHz for pretreatment for 6 minutes. After cooling, the straw fiber porosity was measured to be 22% higher.

[0058] Straw pretreatment: Activated straw was crushed to a particle size of 0.8 mm, and a composite pretreatment agent with a mass fraction of 4% (40 g / L) was added. The composite pretreatment agent was composed of a mixture of citric acid and oxalic acid (20 g / L). Simultaneously, nano-TiO2 with a mass fraction of 0.2% (0.2 g / L) was added. This nano-TiO2 was modified by KH550 at 65℃ for 1.2 hours. 1800 mL of deionized water was added, and the mixture was reacted at 85℃ and a stirring rate of 140 rpm for 2.5 hours. The pretreated straw was obtained by vacuum filtration.

[0059] Lignocellulose extraction: A 1.5% (w / w) alkaline protease solution with an activity of 11000 U / g was added to the pretreated straw, at a concentration of 12% of the straw mass. Enzymatic hydrolysis was carried out at 48℃ and a stirring speed of 90 rpm for 1.8 hours. Then, a 2.5% (w / w) sodium hydroxide solution and 0.08% (w / w) chitosan with a viscosity of 130 mPa·s were added, at a concentration of 0.8% of the straw mass. The mixture was kept at 65℃ and a stirring speed of 90 rpm for 2.2 hours. After filtration, the mixture was washed with deionized water until pH 7.0 and dried at 75℃ to obtain 505 g of lignocellulose.

[0060] Cellulose saccharification: Lignocellulose was mixed with a compound saccharifying enzyme (0.8% by mass), which consisted of 24g of cellulase and 8g of hemicellulase. Simultaneously, 0.04% by mass of the ionic liquid [Emim]Ac was added, with a mass ratio of 1:7 to lignocellulose. The solid-liquid ratio was controlled at 1:9, and deionized water was added to adjust the pH to 4.8. Saccharification was carried out at 52℃ for 3.5 hours, with stirring for 5 minutes every hour during the process. The resulting glucose solution was obtained by filtration.

[0061] Glucose solution purification: The glucose solution is filtered through a 0.12μm ceramic membrane at an operating pressure of 0.12MPa to remove protein impurities with a molecular weight of 700Da, and the permeate is collected.

[0062] Lactic acid fermentation: 0.2% (w / w) of a fermentation aid, composed of 8g yeast extract and 16g ammonium sulfate, was added to the purified glucose solution. Simultaneously, 0.02% (w / w) of graphene quantum dots with a surface carboxyl group content of 5.5 wt% was added. 7% lactic acid bacteria, a 1:1 (w / w) mixture of *Lactobacillus delbrueckii* and *Lactobacillus plantarum*, were inoculated. Anaerobic fermentation was carried out at 36℃ and pH 6.2 for 55 hours, with nitrogen gas introduced during the fermentation process at a flow rate of 0.6 L / min. The resulting lactic acid solution was obtained by filtration.

[0063] Preparation of lactide: Pure lactic acid was added to an oligomerization reactor and oligomerized under vacuum at 125℃ and -0.095MPa for 2.5 hours. 0.08% (by weight of pure lactic acid) of a composite catalyst (6.4g stannous octoate + 1.6g zinc oxide) was added, and the temperature was raised to 190℃ while maintaining the same vacuum. The pyrolysis and cyclization reaction was carried out for 7 hours, and the crude lactide was collected by condensation. The crude lactide was added to 95% ethanol at a solid-liquid ratio of 1:6, kept at 22℃ for 1 hour, filtered, and recrystallized twice. Then, it was dried at 85℃ and -0.09MPa for 5 hours to obtain a high-purity lactide product with 99.5%.

[0064] Polylactic acid polymerization: High-quality lactide is mixed with 0.06% by mass of a composite catalyst (prepared in the same way as lactide), 0.05% by mass of organically modified montmorillonite, and 0.02% by mass of antioxidant 1010, and added to a polymerization reactor; preheated at 125°C and vacuum degree -0.09MPa for 1.5 hours, then heated to 165°C and maintained at vacuum degree, and polymerized at a stirring rate of 30rpm for 10 hours to obtain polylactic acid (PLA).

[0065] Example 3

[0066] Straw activation: 1000g of rice straw was placed in a microwave reactor, and the power was set to 450W and the frequency to 2450MHz for pretreatment for 9 minutes. After cooling, the straw fiber porosity was measured to have increased by 28%.

[0067] Straw pretreatment: Activated straw was crushed to a particle size of 1.2 mm, and a 6% (w / w) composite pretreatment agent was added. The composite pretreatment agent consisted of a mixture of 60 g citric acid and 30 g oxalic acid. Simultaneously, 0.4% (w / w) of nano-TiO2 was added. This nano-TiO2 was modified with KH550 at 75℃ for 1.8 hours. 2200 mL of deionized water was added, and the mixture was reacted at 95℃ and a stirring rate of 160 rpm for 3.5 hours. The pretreated straw was obtained by vacuum filtration.

[0068] Lignocellulose extraction: A 2.5% (w / w) alkaline protease solution with an activity of 14000 U / g was added to the pretreated straw, at a concentration of 18% of the straw mass. Enzymatic hydrolysis was carried out at 52℃ and a stirring speed of 110 rpm for 2.2 hours. Then, a 4% (w / w) sodium hydroxide solution and 0.15% (w / w) chitosan with a viscosity of 180 mPa·s were added, at a concentration of 1.2% of the straw mass. The mixture was kept at 72℃ and a stirring speed of 110 rpm for 2.8 hours. After filtration, the mixture was washed with deionized water until pH 7.0 and dried at 85℃ to obtain 535 g of lignocellulose.

[0069] Cellulose saccharification: Lignocellulose was mixed with a 1.2% (w / w) complex saccharifying enzyme, which consisted of 36g of cellulase and 12g of hemicellulase. Simultaneously, 0.06% (w / w) of the ionic liquid [Emim]Ac was added, with a mass ratio of 1:9 to lignocellulose. The solid-liquid ratio was controlled at 1:11, and deionized water was added to adjust the pH to 5.2. Saccharification was carried out at 58℃ for 4.5 hours, with stirring for 5 minutes every hour during the process. The resulting glucose solution was obtained by filtration.

[0070] Glucose solution purification: The glucose solution is filtered through a 0.18μm ceramic membrane at an operating pressure of 0.18MPa to remove protein impurities with a molecular weight of 900Da, and the permeate is collected.

[0071] Lactic acid fermentation: A fermentation aid of 0.4% (w / w) was added to the purified glucose solution. The fermentation aid consisted of a mixture of 12g yeast extract and 24g ammonium sulfate. Simultaneously, 0.04% (w / w) of graphene quantum dots with a surface carboxyl group content of 6.5 wt% was added. 9% lactic acid bacteria (a 1:1 mixture of *Lactobacillus delbrueckii* and *Lactobacillus plantarum*) were inoculated. Anaerobic fermentation was carried out at 40℃ and pH 6.8 for 65 hours, with nitrogen gas introduced during the fermentation process at a flow rate of 0.9 L / min. The resulting lactic acid solution was obtained by filtration.

[0072] Add 9% activated carbon to a lactic acid solution and adsorb for 1.8 hours at 75℃ and a stirring speed of 120 rpm. Filter to obtain a preliminarily purified lactic acid solution. Concentrate the solution to 55% lactic acid by mass at 45℃ and a vacuum of -0.09 MPa. After concentration, add 0.8% lactic acid seed crystals (0.2 mm particle size) by mass and stir for 30 minutes. Then, use a two-stage cooling program: first, cool from room temperature to 22℃ at a rate of 4℃ / h (at which point the solution is supersaturated by 1.3 times), and then cool to 6℃ at a rate of 9℃ / h. Keep at this temperature and let stand for 5 hours to allow lactic acid to crystallize. Separate the crystals from the mother liquor by centrifugation. Wash the crystals three times with deionized water at 7℃, with each wash using 14% of the crystal mass of water. Finally, dry at 45℃ and a vacuum of -0.09 MPa for 2.5 hours to obtain pure lactic acid.

[0073] Preparation of lactide: Pure lactic acid was added to an oligomerization reactor and oligomerized under vacuum at 135℃ and -0.1MPa for 3.5 hours. 0.15% (by weight of pure lactic acid) of a composite catalyst (12g stannous octoate + 3g zinc oxide) was added, and the temperature was raised to 200℃ while maintaining the same vacuum. The pyrolysis and cyclization reaction was carried out for 8 hours, and the crude lactide was collected by condensation. The crude lactide was added to 95% ethanol at a solid-liquid ratio of 1:8, kept at 20℃ for 1 hour, filtered, and recrystallized three times. Then, it was dried at 90℃ and -0.09MPa for 6 hours to obtain a high-purity lactide product with 99.7%.

[0074] Polylactic acid polymerization: High-quality lactide is mixed with 0.25% by mass of a composite catalyst (prepared in the same way as lactide), 0.08% by mass of organic modified montmorillonite, and 0.03% by mass of antioxidant 1010, and added to a polymerization reactor; preheated at 130°C and vacuum degree -0.09MPa for 2 hours, then heated to 170°C and maintained at vacuum degree, and polymerized at a stirring rate of 30rpm for 12 hours to obtain polylactic acid PLA.

[0075] Comparative Example

[0076] Straw pretreatment: Take 1000g of unactivated corn straw and crush it to a particle size of 1mm. Add 5% sodium hydroxide solution and 2000mL of deionized water. React at 100℃ and 150rpm for 4 hours. Vacuum filter to obtain pretreated straw.

[0077] Lignocellulose extraction: A 2% (w / w) alkaline protease solution with an activity of 12000 U / g was added to the pretreated straw, and the amount added was 15% of the straw mass. Enzymatic hydrolysis was carried out at 50℃ and a stirring speed of 100 rpm for 2 hours. Then, a 3% (w / w) sodium hydroxide solution was added, and the mixture was kept at 70℃ and a stirring speed of 100 rpm for 2.5 hours. After filtration, the mixture was washed with deionized water until the pH reached 7.0, and dried at 80℃ to obtain 382g of lignocellulose.

[0078] Cellulose saccharification: Lignocellulose was mixed with 1% (w / w) of cellulase, which had an enzyme activity of 5000 U / g. The solid-liquid ratio was controlled at 1:8, and deionized water was added to adjust the pH to 5.0. Saccharification was carried out at 60°C for 6 hours, and the glucose solution was obtained by filtration.

[0079] Lactic acid fermentation: Add 0.3% yeast extract to glucose solution, inoculate with 8% Lactobacillus delbrueckii, and anaerobic ferment for 60 hours at 38℃ and pH 6.5. Nitrogen gas is introduced during fermentation at a flow rate of 0.8 L / min, and the lactic acid solution is obtained by filtration.

[0080] Lactic acid purification: Add 8% activated carbon to the lactic acid solution and adsorb for 1.5 hours at 70℃ and a stirring speed of 120 rpm. Filter to obtain a preliminarily purified lactic acid solution. Concentrate the solution to 55% lactic acid by mass at 45℃ and a vacuum of -0.085 MPa. Control the cooling rate to 6℃ / h, cool from room temperature to 8℃, and keep it at this temperature for 5 hours to allow the lactic acid to crystallize. Centrifuge to separate the crystals from the mother liquor. Wash the crystals twice with deionized water at 7℃, with each wash using 13% of the crystal mass of water. Finally, dry at 45℃ and a vacuum of -0.085 MPa for 2.5 hours to obtain pure lactic acid.

[0081] Preparation of lactide: Pure lactic acid was added to a reaction vessel and vacuum oligomerized for 4 hours at 140℃ and -0.09MPa. 0.2% by mass of pure lactic acid was added as a single catalyst (stannous octoate), the temperature was raised to 210℃ and the same vacuum was maintained, and the pyrolysis and cyclization reaction was carried out for 8 hours. Crude lactide was collected by condensation. The crude lactide was simply recrystallized once with 95% ethanol (solid-liquid ratio 1:4, kept at 25℃ for 0.5 hours), and dried at 80℃ and normal pressure for 3 hours to obtain crude lactide with a purity of 95.5%.

[0082] Polylactic acid polymerization: Crude lactide was mixed with 0.15% by mass of a single catalyst (stannous octoate) and added to a polymerization reactor; the mixture was preheated at 130°C and vacuum degree -0.09MPa for 2 hours, then heated to 190°C and vacuum degree -0.1MPa, and stirred at 30rpm for 14 hours to obtain polylactic acid PLA.

[0083] The performance test results of the examples and comparative examples are shown in the table below:

[0084] Table 1

[0085]

[0086] The key parameters for straw pretreatment and enzymatic hydrolysis in the examples and comparative examples are compared in the table below:

[0087] Table 2

[0088] Test item Example 1 Example 2 Example 3 Comparative example Lignin removal rate after straw pretreatment 68％ 65％ 72％ 42％ Reducing sugar concentration in enzymatic hydrolysate 85 g / L 82 g / L 88 g / L 55 g / L Enzymatic hydrolysis efficiency 93％ 91％ 95％ 68％ COD value of pretreatment wastewater 320 mg / L 350 mg / L 300 mg / L 850 mg / L

[0089] As can be seen from the data in Tables 1 and 2, compared with traditional methods, the method of the present invention significantly improves the extraction rate of lignocellulose, the conversion rate of glucose, the removal rate of lignin, and the enzymatic hydrolysis efficiency in terms of raw material conversion and process efficiency, and significantly increases the reducing sugar concentration of the enzymatic hydrolysate. In terms of product quality, the purity of lactic acid, the number-average molecular weight of PLA, the tensile strength, and the thermal decomposition temperature are all significantly optimized. In terms of environmental protection, the COD value of the pretreated wastewater is significantly reduced, and the performance fluctuations among the examples are small and the stability is strong, which fully verifies the advanced nature of the entire process of the present invention in terms of resource utilization, product quality, and environmental economy.

[0090] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing polylactic acid (PLA) using straw lignocellulose, characterized in that, Includes the following steps: S1, Straw pretreatment: Crush straw to a particle size of 0.5-2mm, add 3-8% by mass of composite pretreatment agent and 0.1-0.5% by mass of nano TiO2, stir and react at 80-100℃ for 2-4 hours, and filter to obtain pretreated straw; S2, Lignocellulose extraction: Add 1-3% by mass of alkaline protease solution to pretreated straw, enzymatically hydrolyze at 45-55℃ for 1.5-3 hours, then add sodium hydroxide solution and chitosan, keep warm at 60-75℃ for 2-3 hours, filter, wash until neutral, and dry to obtain lignocellulose; S3, Cellulose saccharification: Lignocellulose is mixed with 0.5-2% by mass of a compound saccharifying enzyme and 0.02-0.1% by mass of an ionic liquid [Emim]Ac, with the solid-liquid ratio controlled at 1:8-12. Saccharification is carried out at 50-60℃ and pH 4.5-5.5 for 3-6 hours, and the glucose solution is obtained by filtration. S4, Lactic acid fermentation: Add 0.1-0.5% fermentation aid and 0.01-0.05% graphene quantum dots to glucose solution, inoculate with 5-10% lactic acid bacteria, and anaerobic ferment at 35-42℃ and pH 6.0-7.0 for 48-72 hours. Filter to obtain lactic acid solution. S5, Lactic acid purification: Add 5-10% activated carbon to the lactic acid solution, stir and adsorb at 60-80℃ for 1-2 hours, and filter to obtain a preliminarily purified lactic acid solution. The solution was concentrated to a lactic acid mass fraction of 50-60% at 40-50℃ and a vacuum of -0.08 to -0.09 MPa. The cooling rate was controlled at 5-8℃ / h, and the solution was slowly cooled from room temperature to 5-10℃. The solution was kept at this temperature and allowed to stand for 4-6 hours to allow lactic acid crystals to precipitate. The crystals were separated from the mother liquor by centrifugation. The crystals were washed 2-3 times with deionized water at 5-10℃, with each wash using 10-15% of the crystal mass of water. Finally, the solution was dried at 40-50℃ and a vacuum of -0.08 to -0.09 MPa for 2-3 hours to obtain pure lactic acid with a purity ≥99.8%. S6, Preparation of lactide: Pure lactic acid is added to a reaction vessel and vacuum oligomerized for 3-5 hours at 130-150℃ and a vacuum degree of -0.095 to -0.1MPa; then, 0.1-0.3% (by mass) of a composite catalyst of pure lactic acid is added, the temperature is raised to 180-200℃ and the above vacuum degree is maintained, and the pyrolysis and cyclization reaction is carried out for 6-8 hours. Crude lactide is collected by condensation; the crude lactide is recrystallized 2-3 times with 95% ethanol and then dried for 4-6 hours at 80-90℃ and a vacuum degree of -0.09MPa to obtain high-purity lactide with a purity ≥99.5%; S7, Polylactic Acid Polymerization: Powdered lactide is mixed with 0.08-0.25% by mass of a composite catalyst, 0.03-0.1% by mass of montmorillonite, and 0.01-0.03% by mass of antioxidant 1010. The mixture is preheated at 120-130°C and a vacuum of -0.09 to -0.1 MPa for 1-2 hours, then heated to 160-170°C and maintained at the above vacuum, and polymerized at a stirring rate of 20-40 rpm for 8-12 hours to obtain polylactic acid (PLA).

2. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, It also includes straw activation: placing the straw in a microwave reactor and pre-treating it for 5-10 minutes at a power of 300-500W and a frequency of 2450MHz to increase the porosity of the straw fiber by 20-30% before crushing.

3. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, It also includes sugar solution purification: the glucose solution is filtered through a 0.1-0.2μm ceramic membrane, the operating pressure is controlled at 0.1-0.2MPa, protein impurities with a molecular weight of 500-1000Da are retained, and the permeate is collected as fermentation raw material.

4. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, The nano-TiO2 in S1 is anatase type, with a particle size of 10-30 nm and a specific surface area of ​​50-80 m². 2 / g, modified with silane coupling agent KH550 before use, at a temperature of 60-80℃ for 1-2 hours; the composite pretreatment agent is a mixture of citric acid and oxalic acid in a mass ratio of 2:

1.

5. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, In S2, the degree of deacetylation of chitosan is not less than 90%, the viscosity is 100-200 mPa·s, the amount added is 0.5-1.5% of the mass of pretreated straw, and the stirring speed is controlled at 80-120 rpm during the heat preservation process.

6. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, The purity of [Emim]Ac in S3 is not less than 98%, the water content is not more than 0.5%, and the mass ratio of [Emim]Ac to lignocellulose is 1:5-10. During the saccharification process, the mixture is stirred once every 1 hour for 5 minutes each time. The compound saccharifying enzyme is composed of cellulase and hemicellulase in a mass ratio of 3:

1.

7. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, The graphene quantum dots in S4 have a particle size of 2-10 nm, a surface carboxyl content of not less than 5 wt%, an excitation wavelength of 360-400 nm, and a fluorescence quantum yield of not less than 30%. Nitrogen gas needs to be introduced during fermentation to maintain an anaerobic environment, with a nitrogen flow rate of 0.5-1 L / min. The fermentation aid is a mixture of yeast extract and ammonium sulfate in a mass ratio of 1:

2.

8. The method for preparing polylactic acid (PLA) from straw lignocellulose according to claim 1, characterized in that, The crystallization step described in S5 adopts a two-stage cooling procedure: first, the solution is cooled from room temperature to 20-25°C at a rate of 3-5°C / h; when the supersaturation of the solution reaches 1.2-1.5 times, it is then cooled to 5-8°C at a rate of 8-10°C / h; and 0.5-1% by mass of lactic acid seed crystals are added to the concentrated lactic acid solution in advance. The seed crystals are added 30 minutes before the first cooling and stirred to disperse evenly.

9. The method for preparing polylactic acid (PLA) from straw lignocellulose according to claim 1, characterized in that, S7 contains sodium-based montmorillonite with an interlayer spacing of 1.5-2.0 nm and a cation exchange capacity of 80-120 mmol / 100g. It is organically modified with hexadecyltrimethylammonium bromide at a temperature of 80-90℃ for 2-4 hours.

10. The method for preparing polylactic acid (PLA) using straw lignocellulose according to claim 1, characterized in that, The S7 polymerization stage adopts a gradient heating mode: 160℃ for 2 hours, 170℃ for 3 hours, and 180℃ for 1-5 hours. The vacuum degree of each stage is increased by 0.005MPa compared with the previous stage, and the stirring rate is controlled at 20-40rpm. The composite catalyst is composed of stannous octoate and zinc oxide in a mass ratio of 4:1.

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