A method for extracting lignin using a mixed acid solution of oxalic acid and maleic acid

Abstract

The present invention belongs to the technical field of resource utilization of lignocellulosic biomass, and particularly relates to a method for extracting lignin using a mixed aqueous solution of oxalic acid and maleic acid. The method comprises the following steps: uniformly loading a lignocellulosic raw material of suitable particle size into a flow reactor, heating the reactor to 120-150°C, then pumping a 120-150°C mixed aqueous solution of oxalic acid and maleic acid into the flow reactor via a pipeline using a mechanical pump to dissolve the lignin in the lignocellulosic raw material; after the acid solution is completely pumped out, pumping in warm water 10-50 times the mass of the lignocellulosic raw material to wash the remaining acid solution to obtain a washing solution; then diluting the acid solution containing dissolved lignin with the washing solution to precipitate the lignin; then performing solid-liquid separation to obtain the lignin and the diluted acid solution; and finally washing the lignin with water and drying to obtain a lignin product. The method is highly operable, does not use or produce any toxic or hazardous substances during the process, and is highly amenable to industrial production.

Description

Technical Field

[0001] The invention belongs to the technical field of resource utilization of lignocellulose biomass, and particularly relates to a method for extracting lignin with a mixed acid solution of oxalic acid and maleic acid. Background Art

[0002] Biomass is currently the only renewable resource that can be directly converted into liquid fuels and chemicals, with lignocellulose being its primary form. The Earth produces approximately 150 billion tons of lignocellulose annually through photosynthesis, of which 15-30% is lignin. Lignin can be modified and processed into a variety of materials and chemicals, particularly single-ring compounds that can replace petroleum-based aromatic compounds. Currently, industrial lignin primarily comes from the papermaking and bioethanol industries. This lignin often undergoes extensive pretreatment, altering its structure and undergoing significant condensation. Ether and lipid bonds break and new C-C bonds form, making its subsequent conversion into high-value chemicals difficult. Therefore, developing new, green pretreatment methods with strong economic viability is crucial for the high-value utilization of lignin.

[0003] In recent years, scholars have developed many methods for extracting lignin, among which the most well-known ones are the low-boiling alcohol method, the high-boiling alcohol method, the organic acid method, the ionic liquid method, the deep eutectic flux method, and the hydrated molten salt method. However, based on actual industrial application considerations, some methods are not conducive to large-scale industrial application due to the high price of reagents, such as the ionic liquid method and the deep eutectic flux method. Some methods are difficult to recover in practice, and the application cost is still high, such as the high-boiling alcohol method and the hydrated molten salt method. The low-boiling alcohol method (methanol, ethanol) and the organic acid method (formic acid, acetic acid) are the more in-depth and comprehensive lignin extraction methods, and are also often used as a pulping method. However, the low-boiling alcohol method and the organic acid method also have the risk of volatility and explosion due to their high vapor pressure, and the equipment used needs to be strictly sealed. In addition, low-boiling alcohol generally needs to be in a certain proportion with water to extract lignin, and the alcohol and acid recovered by evaporation are often mixed with a certain proportion of water due to azeotropy with water, resulting in different ratios of alcohol or acid to water in the initial treatment liquid and the recovered liquid, requiring further distillation and purification, which consumes high energy.

[0004] Recently, Professor Junyong Zhu of the U.S. Department of Agriculture invented a method for extracting lignin using maleic acid (Green Chemistry, 2020, 22 (5), 1605-1617). They extracted birch lignin under normal pressure and close to 100°C (≤120°C) and obtained lignin with better structure and lighter color, opening up a new direction for dicarboxylic acid extraction of lignin from lignocellulose. Maleic acid is a green reagent because it is relatively cheap, has stable performance, is non-toxic and pollution-free, and can be recycled during the process, which can further reduce production costs. However, this method has a low yield when extracting lignin with a better structure, and contains more carbohydrates, which requires further optimization and improvement. Another disadvantage of this method for extracting lignin is that when extracting lignin with a better structure, the lignin is difficult to settle after the acid is diluted, making it difficult to effectively separate the lignin and the acid circulation is poor.

[0005] In order to solve the above problems, this patent proposes a new method for extracting a large amount of lignin with better structure from lignocellulose. By adding a certain proportion of highly acidic oxalic acid to increase the acidity of the acid solution, the bond between lignin and carbohydrates can be effectively broken to obtain lignin with higher purity. The reduction of carbohydrates is also conducive to the precipitation of lignin from the acid solution, which facilitates the recovery of the acid solution. This patent uses the flow through reaction process to reduce the reaction time between the acid solution and lignin, reduce the possibility of lignin condensation, and increase the yield of lignin by increasing the extraction temperature. The proposal of this patent can accelerate the industrialization of the method of extracting lignin with dicarboxylic acid and realize the large-scale extraction of high-quality lignin. Summary of the Invention

[0006] In order to solve the problems of low yield, incomplete lignin precipitation and low lignin purity when maleic acid is used alone to extract lignin, the primary purpose of the present invention is to use oxalic acid, which is a dicarboxylic acid like maleic acid, to enhance the extraction of lignin by maleic acid. Without significantly increasing the difficulty of the lignin extraction process, the extraction yield and purity of lignin are greatly improved, and the structure of lignin is better maintained.

[0007] The purpose of the present invention is achieved through the following solutions:

[0008] A method for extracting lignin using a mixed aqueous solution of oxalic acid and maleic acid, comprising the following steps: loading lignocellulose raw material particles into a hollow cylindrical (e.g., cylindrical) flow reactor (the raw material loading amount is 80-100% of the volume of the hollow cavity in the reactor), heating the reactor to a temperature of 120-150°C, and then using a mechanical pump to pump a mixed aqueous solution of oxalic acid and maleic acid at 120-150°C (the oxalic acid concentration is 1-10 wt%, the maleic acid concentration is 30-60 wt%, and the remainder is water) into the flow reactor through a pipeline to dissolve lignin in the lignocellulose raw material, and the acid solution flowing out of the reactor Condensed water is used to cool the temperature to 20-90°C to obtain an acid solution containing lignin. The amount of acid solution pumped in is controlled to be 8-30 times the mass of the cellulose fiber, and the volume of the acid solution pumped in per minute is 1-20% of the volume of the hollow cavity in the reactor. After the acid solution is pumped out, warm water with a volume of 10-50 times the mass of the cellulose fiber is pumped in to wash the residual acid solution. The water temperature is 50-90°C to obtain a washing solution. The washing solution is mixed with the acid solution containing lignin, and the acid solution containing lignin is diluted with the washing solution to precipitate lignin. The lignin and the diluted acid solution are then separated by solid-liquid separation. The lignin is washed with water and then dried to obtain a lignin product.

[0009] The flow reactor is cylindrical with upper and lower ends open, and its axis is placed perpendicular to the horizontal plane. The upper and lower ends of the reactor are both equipped with filter screens to prevent the leakage of lignocellulose particles between them.

[0010] The wood cellulose raw materials include one or more of wood cellulose raw materials and non-wood cellulose raw materials; the wood cellulose raw materials include one or more of pine, eucalyptus, poplar, ash, sea buckthorn, cypress, fir, and birch; the non-wood cellulose raw materials include one or more of bamboo, corn cobs, corn stalks, wheat straw, bagasse, rice straw, rice husks, and peanut shells; the particle sizes of wood raw materials and bamboo are less than 2 cm respectively, and the particle size of non-wood raw materials (excluding bamboo) is less than 5 cm.

[0011] The aspect ratio of the cylindrical flow reactor is 2 to 15:1, preferably 3 to 8:1 (ratio of length to inner diameter).

[0012] The filling volume of the lignocellulose raw material in the circulation reactor is 80-100%, preferably 85-95%, to improve the efficiency of the reactor.

[0013] The reaction temperature of the reactor is 120-150°C, preferably 125-140°C; the method for heating the reactor can be one or both of heating resistance wires or heating jackets with heating medium (the heating medium is one or both of water vapor and thermal oil).

[0014] The temperature of the mixed acid pumped into the reactor is 120-150° C., preferably 125-140° C.; the concentration of oxalic acid in the mixed acid is 1-10wt%, preferably 3-8wt%, and the concentration of maleic acid is 30-60wt%, preferably 40-50wt%.

[0015] The mass of the acid liquid pumped in is 8 to 30 times the mass of the lignocellulose, preferably 10 to 15 times; the volume of the acid liquid pumped in per minute is 1 to 20% of the volume of the hollow cavity in the reactor, preferably 2 to 10%.

[0016] The acid solution flowing out of the reactor is cooled to 20-90°C with condensed water, preferably to 40-80°C.

[0017] The mass of the warm water used to wash the residual acid in the lignocellulose is 10 to 50 times, preferably 15 to 30 times, of the initial mass of the lignocellulose; the temperature of the warm water is 50 to 90°C, preferably 60 to 80°C.

[0018] The diluted acid solution can be recycled after being concentrated to the initial acid solution concentration.

[0019] The mechanism of the present invention is:

[0020] Dicarboxylic acid can solubilize lignin when the link between lignin and carbohydrate is broken due to its acidity and hydrophilicity, and the acid solution containing lignin obtained can both be precipitated out lignin after dilution. What the method for dicarboxylic acid extraction lignin in early stage uses is independent maleic acid, and because the acidity of maleic acid is weaker, it is not good to break the link between lignin and carbohydrate, so that when obtaining the lignin with good structure preservation (general treatment conditions are weaker), lignin contains more carbohydrate. By adding oxalic acid with stronger acidity, the link between lignin and carbohydrate can be broken more effectively, thereby improving the yield of lignin, and obtaining the higher lignin of purity. If oxalic acid is changed into inorganic strong acids such as hydrochloric acid and sulfuric acid, then serious condensation of lignin will be caused, and the lignin with good structure preservation will not be obtained.

[0021] Compared with the prior art (using maleic acid extraction alone), the present invention has the following advantages and beneficial effects:

[0022] (1) The lignin extracted by the present invention has a high yield and high purity, which is conducive to subsequent high-value conversion.

[0023] (2) The present invention has strong operability, good reagent recycling, and is suitable for industrial production.

[0024] The lignin extracted by the present invention can well retain the original structure of lignin and present a light yellow to pink color, which can greatly improve the subsequent utilization value of lignin. The present invention has strong operability, does not use or produce toxic and harmful substances during the process, and is very easy to industrialize. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The birch lignin was extracted by a mixed aqueous solution of oxalic acid and maleic acid through a flow-through reactor. The acid solution was diluted 3 times and allowed to stand for 30 minutes, corresponding to the initial extraction in Example 2. It can be seen that the obtained lignin is lighter in color, and the lignin in the acid solution is almost completely precipitated. The acid solution can be easily recovered after separation and concentration.

[0026] Figure 2 Schematic diagram of the cylindrical flow reactor used in the examples, with filter screens and flanges installed at both ends of the reactor, which are detachable. DETAILED DESCRIPTION

[0027] To further illustrate the present invention, several specific implementation examples are given below, but the present invention is not limited to these examples. Example 1 is a comparative example of Example 2, Example 7 is a comparative example of Example 8, and Example 12 is a comparative example of Example 13.

[0028] Since the design and manufacture of flow reactors are relatively time-consuming, the following examples use three hollow cylindrical flow reactors A with openings at both ends (the inner cavity volume of the cylindrical flow reactor is 188.5 cm 3 The hollow part of the reactor has a diameter of 40 mm and a length of 150 mm. The lower open end is connected to an inlet capillary tube with a diameter of 5 mm and a length of 1000 mm. The upper open end is connected to an outlet capillary tube with a diameter of 3 mm and a length of 500 mm. The upper and lower ports of the reactor are both equipped with 400-mesh filters. B (the inner cavity volume of the cylindrical flow reactor is 565.5 cm 3 The hollow part of the reactor has a diameter of 60 mm and a length of 200 mm. The lower open end is connected to an inlet capillary tube with a diameter of 6 mm and a length of 1000 mm. The upper open end is connected to an outlet capillary tube with a diameter of 6 mm and a length of 500 mm. The upper and lower ports of the reactor are both equipped with 400-mesh filters) and C (the inner cavity volume of the cylindrical flow reactor is 106 cm 3 The hollow part inside the reactor has a diameter of 30 mm and a length of 150 mm. The lower open end is connected to an inlet capillary tube with a diameter of 3 mm, and the upper open end is connected to a capillary tube with a length of 1000 mm. The outlet capillary tube has a diameter of 3 mm and a length of 500 mm. The upper and lower ports of the reactor are both equipped with a 400-mesh filter. The schematic diagram of the three reactors is as follows Figure 2As shown, they are all made of 304 stainless steel, axially positioned perpendicular to the horizontal plane. O-rings are installed at the upper and lower openings of the cylindrical flow reactor and are connected to the inlet and outlet capillaries respectively by flange connections. They can withstand a pressure of approximately 1 MPa. The diameters mentioned above are all inner diameters. In the following embodiments, electric heating belts are installed on the outer wall of the reactor for heating and insulation. The acid solution is first heated to 80°C by a magnetic heating and stirring device (German IKA IKA magnetic electric stirrer C-MAG), then raised to a predetermined temperature through a heating pipe and enters the flow reactor through the inlet at the lower end. Warm water is heated and insulated by a magnetic heating and stirring device (German IKA IKA magnetic electric stirrer C-MAG) and enters the flow reactor through the inlet at the lower end.

[0029] Example 1: 25 g of birch wood powder with a 40-mesh filter was evenly charged into a flow reactor A (the charging volume was 85% of the reactor volume), and the temperature of the reactor was raised to 120° C. by an electric heating belt. Then, 250 g of an 80° C. maleic acid aqueous solution (maleic acid concentration was 50 wt%) was heated to 120° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 10 mL / min to contact with the wood powder to dissolve lignin. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 80° C. containing lignin. After the acid solution was pumped out, 500 g of The residual acid solution was washed with warm water at 80°C, and the obtained washing solution was added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution were then separated by centrifugation. The lignin was washed with clean water and then freeze-dried to obtain a lignin product. The diluted acid solution was concentrated to the initial acid solution concentration and then recycled twice. The process and operating conditions were the same as the first time.

[0030] Example 2: 25 g of birch wood powder with a 40-mesh filter was evenly charged into a flow reactor A (the charging volume was 85% of the reactor volume), and the temperature of the reactor was raised to 120° C. by an electric heating belt. Then, 250 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 5 wt % and 45 wt %, respectively) at 80° C. was heated to 120° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 10 mL / min to contact with the wood powder to dissolve lignin. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 80° C. containing lignin. After the acid solution was pumped out, 500 g of The residual acid solution was washed with warm water at 80°C, and the obtained washing solution was added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution were then separated by centrifugation. The lignin was washed with clean water and then freeze-dried to obtain a lignin product. The diluted acid solution was concentrated to the initial acid solution concentration and then recycled twice. The process and operating conditions were the same as the first time.

[0031] Example 3: 30 g of 40-mesh poplar wood powder was evenly charged into a flow reactor A (the charging volume was 95% of the reactor volume), and the temperature of the reactor was raised to 130° C. by an electric heating belt. Then, 400 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 10 wt % and 40 wt %, respectively) at 80° C. was heated to 130° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 15 mL / min to contact and react with the wood powder. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 85° C. containing lignin. After the acid solution was pumped out, 600 g of oxalic acid and maleic acid were pumped into the flow reactor. The residual acid solution is washed with warm water at 90°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0032] Example 4: 20 g of rice straw with a 10-mesh filter was uniformly charged into a flow reactor A (the charging volume was 90% of the reactor volume), and the temperature of the reactor was raised to 125° C. by an electric heating belt. Then, 180 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 15 wt % and 45 wt %, respectively) at 80° C. was heated to 125° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.), and then pumped into the flow reactor at a rate of 15 mL / min to react with the grass powder. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 75° C. containing lignin. After the acid solution was pumped out, 400 g of oxalic acid was pumped into the flow reactor. The residual acid solution is washed with warm water at 60°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0033] Example 5: 20 g of rice straw with a 10-mesh filter was uniformly charged into a flow reactor A (the charging volume was 90% of the reactor volume), and the temperature of the reactor was raised to 125° C. by an electric heating belt. Then, 180 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 15 wt % and 45 wt %, respectively) at 80° C. was heated to 125° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.), and then pumped into the flow reactor at a rate of 15 mL / min to contact with the grass powder for reaction. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 75° C. containing lignin. After the acid solution was pumped out, 400 g of The residual acid solution is washed with warm water at 70°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0034] Example 6: 30 g of pine wood powder with a 10-mesh filter was uniformly charged into a flow reactor A (the charging volume was 95% of the reactor volume), and the temperature of the reactor was raised to 130° C. by an electric heating belt. Then, 300 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 10 wt % and 45 wt %, respectively) at 80° C. was heated to 130° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 15 mL / min to react with the wood powder. The outlet pipe was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 80° C. containing lignin. After the acid solution was pumped out, 600 g of oxalic acid was pumped into the flow reactor. The residual acid solution is washed with warm water at 50°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0035] Example 7: 50g of 40-mesh bagasse was uniformly loaded into the flow reactor B (the loading volume was 90% of the reactor volume), the temperature of the reactor was increased to 130°C by an electric heating belt, and then 500g of 80°C maleic acid aqueous solution (maleic acid concentration was 50wt%) was pumped into the flow reactor at a speed of 20mL / min with a mechanical pump (HY-1T02 advection pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) through a pipeline to contact the bagasse for reaction, and the outlet pipe was condensed with normal temperature (about 20°C) tap water to obtain an acid solution with a temperature of about 80°C that was dissolved with lignin. After the acid solution was pumped out, 1000g of 80°C warm water was pumped into the residual acid solution, the washings obtained were added to the acid solution that was dissolved with lignin and mixed and diluted to precipitate lignin. The lignin was then washed with clean water and freeze-dried to obtain a lignin product.

[0036] Example 8: 50 g of 40-mesh bagasse was uniformly charged into the flow reactor B (the charging volume was 90% of the reactor volume), and the temperature of the reactor was raised to 130° C. by an electric heating belt. Then, 500 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 3 wt % and 50 wt %, respectively) at 80° C. was heated to 130° C. by a mechanical pump (HY-1T02 advection pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) through a heating pipe. The solution was then pumped into the flow reactor at a rate of 20 mL / min to react with the bagasse. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 80° C. containing lignin. After the acid solution was pumped out, 1000 g of oxalic acid and maleic acid were pumped into the flow reactor. The residual acid solution is washed with warm water at 80°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0037] Example 9: 40 g of wheat straw with a 20-mesh diameter was uniformly charged into the flow reactor B (the loading volume was 85% of the reactor volume), the temperature of the reactor was raised to 125° C. by an electric heating belt, and then 600 g of wheat straw was pumped into the reactor using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.). A mixed acid aqueous solution of oxalic acid and maleic acid (with oxalic acid and maleic acid concentrations of 15 wt% and 40 wt%, respectively) at 80°C is heated to 125°C through a heating pipe and then pumped from the lower end into a circulation reactor at a rate of 30 mL / min to react with wheat straw. The outlet capillary is condensed with tap water at room temperature (about 20°C) to obtain an acid solution with a temperature of about 60°C containing lignin. After the acid solution is pumped out, 1200 g of warm water at 70°C is pumped in to wash the remaining acid solution. The obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate lignin. The lignin and the diluted acid solution are then centrifuged and separated. The lignin is washed with clean water and freeze-dried to obtain a lignin product.

[0038] Example 10: 45 g of corn cobs with a 20-mesh filter were uniformly loaded into a flow reactor B (the loading volume was 90% of the reactor volume), and the temperature of the reactor was raised to 130° C. by an electric heating belt. Then, 800 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 3 wt % and 60 wt %, respectively) at 80° C. was heated to 130° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 25 mL / min to contact and react with the corn cobs. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 50° C. containing lignin. After the acid solution was pumped out, 1600 g of oxalic acid and maleic acid were pumped into the flow reactor. The residual acid solution is washed with warm water at 90°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0039] Example 11: 10 g of birch wood powder that passed 10 mesh was charged into a flow reactor C (the charging volume was 90% of the reactor volume), and the temperature of the reactor was raised to 150° C. by an electric heating belt. Then, 100 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 10 wt % and 40 wt %, respectively) at 80° C. was heated to 150° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 15 mL / min to contact and react with the wood powder. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 90° C. containing lignin. After the acid solution was pumped out, 300 g of The residual acid solution is washed with warm water at 60°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0040] Example 12: 12 g of wheat straw passing through a 20-mesh filter was evenly charged into a flow reactor C (the loading volume was 98% of the reactor volume), and the temperature of the reactor was raised to 135° C. by an electric heating belt. Then, 150 g of an aqueous solution of maleic acid at 80° C. (maleic acid concentration was 50 wt%) was heated to 135° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 12 mL / min to contact the wheat straw for reaction. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 60° C. containing dissolved lignin. After the acid solution was pumped out, 400 g of warm water at 75° C. was pumped into the solution to wash the remaining acid solution. The washing solution was added to the acid solution containing dissolved lignin, mixed and diluted, and lignin was precipitated. The lignin and the diluted acid solution were then centrifuged and separated. The lignin was washed with clean water and freeze-dried to obtain a lignin product.

[0041] Example 13: 12 g of wheat straw that passed 20 mesh was evenly charged into a flow reactor C (the charging volume was 98% of the reactor volume), and the temperature of the reactor was raised to 135° C. by an electric heating belt. Then, 150 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 10 wt % and 40 wt %, respectively) at 80° C. was heated to 135° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor at a rate of 12 mL / min to react with the wheat straw. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 50° C. containing lignin. After the acid solution was pumped out, 400 g of The residual acid solution is washed with warm water at 75°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0042] Example 14: 15 g of corn cobs with a 20-mesh filter were uniformly charged into a flow reactor C (the charging volume was 100% of the reactor volume), and the temperature of the reactor was raised to 130° C. by an electric heating belt. Then, 200 g of a mixed acid solution of oxalic acid and maleic acid (the concentrations of oxalic acid and maleic acid were 5 wt % and 50 wt %, respectively) at 80° C. was heated to 130° C. through a heating pipe using a mechanical pump (HY-1T02 horizontal flow pump, Shanghai Haoyun Instrument Equipment Co., Ltd.) and then pumped into the flow reactor from the lower end at a rate of 8 mL / min to contact and react with the corn cobs. The outlet capillary was condensed with tap water at room temperature (about 20° C.) to obtain an acid solution with a temperature of about 40° C. containing dissolved lignin. After the acid solution was pumped out, 400 g of The residual acid solution is washed with warm water at 80°C, and the obtained washing solution is added to the acid solution containing lignin for mixing and dilution to precipitate the lignin. The lignin and the diluted acid solution are then separated by centrifugation. The lignin is washed with clean water and then freeze-dried to obtain a lignin product.

[0043] The yield of lignin in the above examples is the percentage of the mass of the obtained lignin product to the mass of the lignin in the raw material, and is calculated as follows:

[0044] Lignin yield = mass of extracted lignin product / mass of lignin in raw material × 100%

[0045] The purity of lignin is measured by a two-step acid hydrolysis method. Briefly stated, 0.3 g of lignin is first hydrolyzed with 3 mL of 72 wt% sulfuric acid at 30 ° C for 60 min. The acid solution is then diluted to 4 wt% and hydrolyzed in an autoclave at 121 ° C for 60 min. The solid is filtered and washed and then dried in an oven at 105 ° C for 24 h. The mass is measured as m1. The solid is then calcined in a muffle furnace at 580 ° C for 2 h. The mass is measured as m2. The purity of lignin is calculated as follows:

[0046] Lignin purity = (m1-m2) / 0.3×100%

[0047] The relative content of β-O-4 bonds in lignin is determined by 1 H- 13 The relative content of β-O-4 bonds in the extracted lignin was measured by C2-NMR and recorded as c1. The relative content of β-O-4 bonds in the raw material was recorded as c2. The specific calculation method can be found in the paper (Green Chemistry, 2020, 22(5), 1605-1617). The retention rate of β-O-4 bonds is calculated as follows:

[0048] β-O-4 bond retention rate = c1 / c2×100%

[0049] Table 1. Yield, purity and β-O-4 bond content of extracted lignin

[0050]

[0051] Embodiment 1 is the comparative example of embodiment 2, and it can be seen that when only using maleic acid to extract lignin, adopting flow-through reactor can only extract 30% lignin for the first time, and purity only has 83%, and the retention rate of β-O-4 key has 86%. In the recycling of twice acid solution, the purity of lignin changes little, but the yield of lignin and the retention rate of β-O-4 key all obviously decline. In embodiment 2, 5wt% maleic acid is replaced by oxalic acid, and when keeping the total acid concentration of 50wt% constant, the initial extraction yield of lignin increases to 55%, and the retention rate of β-O-4 key is slightly reduced to 83%. However, in the two rounds of acid solution circulation extraction process, the yield of lignin and the retention rate of β-O-4 key only appear slight decline, and purity does not change significantly. This explanation can obviously improve the extraction yield and purity of lignin after adding oxalic acid, has little influence on the β-O-4 key retention rate, and significantly increases the recycling stability of acid solution.

[0052] Example 7 is a comparative example of Example 8. In Example 8, 3 wt % oxalic acid was added to the 50 wt % maleic acid concentration in Example 7, while other conditions remained unchanged. It can be seen that after adding 3 wt % oxalic acid, the extraction rate of lignin increased from 45% to 62%, the purity increased from 85% to 96%, and the β-O-4 bond content decreased slightly from 85% to 83%. Example 12 is a comparative example of Example 13. In Example 13, the 50 wt % maleic acid concentration in Example 12 was replaced with 40 wt % maleic acid and 10 wt % oxalic acid, while the total acid concentration remained unchanged. It can be seen that after replacing 10 wt % maleic acid with oxalic acid, the extraction rate of lignin increased from 48% to 65%, the purity increased from 84% to 96%, and the β-O-4 bond content decreased slightly from 80% to 78%.

[0053] Examples 3 to 6, 9 to 11, and 14 respectively illustrate the extraction of lignin from different raw materials using mixed acid solutions of oxalic acid and maleic acid under different conditions. The extraction yield of lignin ranged from 50% to 73%, with purities exceeding 90%, and a retention rate of β-O-4 bonds between 58% and 85%. In Example 6, since the substrate was pine wood, which has a relatively dense structure, the lignin extraction yield was only 50%. Compared to the widely used groundwood lignin (generally around 20%), the extraction yield of lignin in this method was significantly improved, and the extraction time was short and the efficiency was high. This method has the potential to be used as a standardized lignin for scientific research, and can also be used as a raw material for the production of high-value chemicals.

[0054] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for extracting lignin from a mixed aqueous solution of oxalic acid and maleic acid, characterized in that The method comprises the following steps: loading lignocellulose raw material particles into a hollow cylindrical flow reactor, wherein the raw material loading amount is 80-100% of the volume of the hollow cavity in the reactor; heating the temperature of the reactor to 120-150°C; then using a mechanical pump to pump a 120-150°C mixed acid solution of oxalic acid and maleic acid, wherein the oxalic acid concentration is 1-10 wt%, the maleic acid concentration is 30-60 wt%, and the remainder is water, into the flow reactor through a pipeline to dissolve lignin in the lignocellulose raw material; the acid solution flowing out of the reactor is cooled to a temperature of 20-90°C with condensed water to obtain an acid solution containing lignin; the pumping amount of the acid solution is controlled to be 8-30 times the mass of the lignocellulose; the volume of the acid solution pumped per minute is 1-20% of the volume of the hollow cavity in the reactor; after the acid solution is pumped out, warm water with a volume of 10-50 times the mass of the lignocellulose is pumped in to wash the residual acid solution; the water temperature is 50-90°C; ° C, to obtain a washing liquid, mixing the washing liquid with an acid solution containing lignin, diluting the acid solution containing lignin with the washing liquid to precipitate lignin, and then performing solid-liquid separation to obtain lignin and the diluted acid solution, washing the lignin with water and then drying to obtain a lignin product.

2. The method according to claim 1, wherein: The volume of acid pumped into the reactor per minute is 2-10% of the volume of the hollow cavity.

3. The method according to claim 1, wherein: The flow reactor is cylindrical with upper and lower ends open, and its axis is placed perpendicular to the horizontal plane. The upper and lower ends of the reactor are both equipped with filter screens to prevent the leakage of lignocellulose particles between them.

4. The method according to claim 1, wherein: The wood cellulose raw materials include one or more of wood cellulose raw materials and non-wood cellulose raw materials; the wood cellulose raw materials include one or more of pine, eucalyptus, poplar, ash, sea buckthorn, cypress, fir, and birch; the non-wood cellulose raw materials include one or more of bamboo, corn cobs, corn stalks, wheat straw, bagasse, rice straw, rice husks, and peanut shells; the particle sizes of the wood cellulose raw materials and bamboo are less than 2 cm respectively, and the particle size of non-wood cellulose raw materials other than bamboo is less than 5 cm.

5. The method according to claim 1, characterized in that: The aspect ratio of the cylindrical flow reactor is 2-15:1, which is the ratio of length to inner diameter.

6. The method according to claim 5, characterized in that: The aspect ratio of the cylindrical flow reactor is 3 to 8:

1.

7. The method according to claim 1, characterized in that: The filling volume of the lignocellulose raw material in the flow reactor is 85-95%.

8. The method according to claim 1, wherein: The reaction temperature of the reactor is 125-140°C.

9. The method according to claim 1, wherein: The temperature of the mixed acid pumped into the reactor is 125-140 ° C; the concentration of oxalic acid in the mixed acid is 3-8 wt%, and the concentration of maleic acid is 40-50 wt%.

10. The method according to claim 1, wherein: The acid solution flowing out of the reactor is cooled to 40-80 ° C with condensed water.

11. The method according to claim 1, wherein: The mass of the warm water used to wash the residual acid in the lignocellulose is 15 to 30 times the mass of the initial lignocellulose; the temperature of the warm water is 60 to 80°C.

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