Application of alkaline eutectic solvent in oxidation pretreatment of straw to realize separation of three elements

Abstract

The invention discloses an application of an alkaline eutectic solvent in oxidation pretreatment of straws to realize separation of three elements. The application method for preparing an alkaline deep eutectic solvent (DES) through reaction of quaternary ammonium salt and an amine / amido compound and pretreating straw with H2O2-Fe2 +-NaOH (Fenton's reagent) as an oxidation system to realize separation of three elements comprises the following steps: firstly, mixing the straw, the DES solvent and an oxidizing agent, reacting for 30-180 minutes at 60-120 DEG C, and then extracting and separating straw residues and lignin. According to the method, the removal rate of straw lignin reaches 85%, the retention rate of cellulose reaches 90% or above, and the obtained residues have huge potential of enzymatic conversion. After the recovered DES solvent is recycled for 10 times, the removal rate of lignin and the retention rate of cellulose still reach 80% and 92% or above. According to the method, straw components can be efficiently separated by using a low-cost solvent at low temperature in a short time, and cellulose components are retained while the removal rate of lignin is increased.

Description

Technical Field

[0001] This invention belongs to the technical field of lignocellulose agricultural and forestry waste resource utilization, specifically involving the preparation of an alkaline eutectic solvent and its application in the separation of three elements in the oxidation pretreatment of straw. Technical Background

[0002] The energy crisis is one of the major challenges facing the world today. Fossil fuels still dominate the global energy mix, with traditional energy sources such as coal, oil, and natural gas remaining the primary sources of global energy supply. However, the continued surge in fossil fuel prices has triggered a new oil crisis, while the ever-increasing energy demand driven by industrialization is accelerating the depletion of these finite resources. Given the non-renewable nature of traditional energy sources like oil, the development and promotion of sustainable alternative energy sources has become an urgent global issue that needs to be addressed.

[0003] Lignocellulosic biomass, derived from agricultural and forestry waste and crops specifically used for energy production, is considered a strategic resource for replacing fossil fuels and producing sustainable bio-based products due to its abundant reserves, high renewability, and lack of reliance on food resources. This type of biomass primarily consists of structural polysaccharides (including cellulose and hemicellulose) and lignin. Cellulose and hemicellulose, as high-molecular-weight polymers, can be hydrolyzed to produce fermentable sugars, which can then be converted into biofuels or platform chemicals. Lignin, as a complex aromatic polymer, has the potential to be used in the production of high-value-added bio-based chemicals or materials. Among various lignocellulosic raw materials, cereal residues such as rice straw, wheat straw, corn straw, and bagasse have become important and preferred raw materials in the field of biorefining due to their widespread availability, high polysaccharide content, and relatively concentrated production methods.

[0004] Enzymatic conversion of carbohydrates is the most promising method for producing monosaccharides and other chemicals. Lignocellulose biomass has a highly complex structure, primarily composed of cellulose, hemicellulose, and lignin. Cellulose, arranged in tightly packed crystalline microfibrils, forms the skeletal structure of plant cell walls; hemicellulose, distributed amorphously around the cellulose microfibrils, acts as a coating and protectant; lignin, a three-dimensional network macromolecule, covalently cross-links the surfaces of cellulose and hemicellulose, further strengthening the mechanical strength and stability of the cell walls. Cellulose molecules form a dense network primarily through hydrogen bonds, while hemicellulose and lignin are linked by ether and ester bonds, forming a complex heteropolymer complex. This multi-scale, tightly interwoven, and chemically bonded structure results in lignocellulose exhibiting strong resistance to degradation and biological resistance. Direct enzymatic hydrolysis and saccharification of natural lignocellulose not only significantly increases the amount of cellulase required but also results in extremely low hydrolysis efficiency, often making it difficult to achieve effective results. Therefore, pretreatment processes are necessary before enzyme treatment to disrupt the robust cell wall structure, reduce cellulose crystallinity, and partially remove or modify the hemicellulose and lignin layers, thereby enhancing enzyme accessibility and increasing sugar yield. Pretreatment has become a key technological step in achieving efficient biorefining of lignocellulose.

[0005] Developing efficient, low-consumption pretreatment technologies that facilitate the high-value utilization of lignin is crucial for realizing the resource utilization of lignocellulose. Traditional pretreatment methods can be categorized into four main types: physical, physicochemical, chemical, and biological. Although existing pretreatment processes are relatively mature, they still generally suffer from bottlenecks such as high energy consumption, high cost, harsh reaction conditions (e.g., high temperature, high pressure, strong corrosion), and the formation of inhibitors and environmental risks. Desiccant solvents (DES) are eutectic systems formed by mixing hydrogen bond donors and acceptors in a specific stoichiometric ratio, creating a stable hydrogen bond network through strong charge delocalization. These solvents offer advantages such as customizable composition, strong solubility, and tunable physicochemical properties. Furthermore, DES are simple to prepare, low in energy consumption, recyclable, and environmentally friendly, making them particularly suitable for large-scale, sustainable pretreatment processes of plant fiber raw materials. New et al. (Process Safety and Environmental Protection 123(2019):190-198) synthesized a neutral DES solvent using choline chloride and urea for pretreatment of oil palm leaves, achieving a lignin removal rate of 11.1%. Further research revealed that the addition of water reduced viscosity, increasing the lignin removal rate to 16.3%. Ma et al. (Molecules 27.22(2022):7955) used a DES system composed of choline chloride and lactic acid to pretreat wheat straw, achieving a lignin removal rate between 9.3% and 81.5%, but with low cellulose retention rates of only 49.9% to 73.6%. This is because while acidic DES can effectively catalyze the cleavage of ether bonds in lignin, its strong acidity also causes non-selective degradation of cellulose, leading to a decrease in cellulose content. Alkaline DES exhibits significantly better swelling effects on lignocellulose than acidic systems. Its alkaline environment selectively cleaves the β-O-4 ether bonds in lignin through nucleophilic reactions, while effectively maintaining the structural integrity of cellulose. To further enhance treatment efficiency, chemical aids are often introduced. These methods strengthen solvent hydrogen bonding interactions and promote substrate mass transfer and accessibility, effectively overcoming the limitations of traditional pretreatment and achieving significant synergistic effects. For example, Yang et al. (Fuel 378(2024):132932) combined the auxiliary effect of NaOH to prepare an alkaline DES pretreatment of rapeseed straw using choline chloride and thiourea. Although the lignin removal rate was low (36.1%), the reaction could proceed at room temperature, indicating that alkaline eutectic solvents have great application potential. Although alkaline DES pretreatment can selectively remove lignin and retain cellulose well, it still faces many challenges. Its pretreatment conditions are often harsh (high temperature, long time), resulting in high energy consumption; solvent costs are high and recycling is difficult, seriously affecting its economic viability; the process may cause hemicellulose loss and increased cellulose crystallinity, which in turn increases the difficulty of subsequent enzymatic hydrolysis. These factors collectively restrict its potential for large-scale application. Summary of the Invention

[0006] To address the shortcomings and limitations of existing technologies, the primary objective of this invention is to provide a method for preparing an alkaline eutectic solvent (DES). This method uses quaternary ammonium salts as hydrogen acceptors and / or amine / amide compounds as hydrogen donors to synthesize an alkaline eutectic solvent. In synergy with Fenton's reagent, this method enables short-term pretreatment of straw under mild conditions, efficiently separating straw components, extracting lignin, and retaining most of the cellulose components.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for preparing an alkaline eutectic solvent (DES) mainly includes the following steps:

[0009] Using quaternary ammonium salts as hydrogen acceptors and amine / amide compounds as hydrogen donors, the hydrogen acceptors and hydrogen donors are mixed and stirred at 60–100 °C until the solution becomes clear, thus obtaining an alkaline eutectic solvent.

[0010] Preferably, the molar ratio of the hydrogen acceptor to the hydrogen donor is 1:2 to 1:6.

[0011] Preferably, the quaternary ammonium salt is at least one of choline chloride, choline bromide, and acetylcholine chloride, and the amine / amide group is at least one of urea, formamide, acetamide, diethanolamine, and ethanolamine.

[0012] Preferably, the stirring reaction time is 0.5 to 1 hour.

[0013] The treated straw is synthesized using an alkaline eutectic solvent under normal pressure, requiring no purification and can be used directly after cooling.

[0014] The above method is used to prepare an alkaline eutectic solvent.

[0015] The application of the above-mentioned alkaline eutectic solvent in the separation of the three elements in the oxidative pretreatment of straw includes the following steps:

[0016] (1) Straw pretreatment: Mix straw with the above-mentioned alkaline eutectic solvent and auxiliary chemical reagent (Fenton reagent) evenly, react at 60-120℃ for 0.5-3h under normal pressure, cool to room temperature, add water and ethanol respectively for solid-liquid separation, the obtained solid component is straw residue rich in carbohydrates, and the liquid component is DES component containing lignin and other degradation components.

[0017] (2) Remove ethanol from the liquid component in step (1), add water to precipitate the lignin dissolved in DES solvent, filter and freeze-dry the filter residue to obtain lignin powder, evaporate and concentrate the filtrate to recover the alkaline eutectic solvent and recycle it.

[0018] (3) Wash and dry the solid residue components from step (1) to obtain straw residue.

[0019] Preferably, the bath ratio of straw and alkaline eutectic solvent in step (1) is 1:5 to 1:20.

[0020] Preferably, the reaction temperature in step (1) is 60 to 100°C.

[0021] Preferably, in step (1), the mixing ratio of straw and solvent refers to the mass to the volume ratio of the liquid.

[0022] Preferably, the straw in step (1) is at least one of corn straw, wheat straw, rice straw, bagasse, and miscanthus.

[0023] Preferably, the concentration of the auxiliary chemical reagent added in step (1) is 6-15 g / L, and the H2O content is 30-50%.

[0024] The composition of straw before and after pretreatment was analyzed according to the standard method of the National Renewable Energy Laboratory (NREL) in the United States.

[0025] Compared with existing pretreatment technologies, the present invention has the following advantages and different effects:

[0026] 1. By using the alkaline eutectic solvent pretreatment of straw as described in this invention, lignin can be efficiently removed from straw in a short time and at a mild temperature, with a lignin removal rate of 70-85% and a cellulose retention rate of over 90%.

[0027] 2. Pretreatment of straw with the alkaline eutectic solvent described in this invention, combined with the synergistic effect of Fenton's reagent, can greatly improve the treatment efficiency.

[0028] 3. This invention uses quaternary ammonium salts as hydrogen acceptors and / or amine / amide compounds as hydrogen donors to prepare DES solvents. The resulting DES solvents can achieve strong dissolution of lignin. The alkaline environment provided by alkaline DES and the oxidation effect of Fenton's reagent can efficiently break the key chemical bonds of lignin. The strong hydrogen bond network and ionic properties of the solvent itself promote the dissolution of the broken lignin fragments, thereby separating them from cellulose. At the same time, the cellulose structural skeleton is protected while dissolving. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0030] Unless otherwise specified, the implementation conditions in this invention embodiment shall be performed under conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0031] Example 1

[0032] (1) Synthesis of alkaline eutectic solvent: Choline chloride and urea are mixed in a molar ratio of 1:2 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain alkaline eutectic solvent.

[0033] (2) Wheat straw pretreatment: 1g of wheat straw, 5mL of the alkaline eutectic solvent from step (1) and 5mL of an aqueous solution containing 6g / L (NaOH, H2O2, FeSO4·7H2O) were mixed evenly and reacted at 60℃ under normal pressure for 1.5h. After the reaction was completed and cooled, 50mL of ethanol was added to dilute the mixture and then filtered to separate the solid components. The solid components were washed with an appropriate amount of H2O. The obtained solid components were wheat straw residues rich in cellulose, and the obtained liquid components were DES components containing dissolved lignin and other degradation products.

[0034] (3) After recovering ethanol from the liquid component obtained in step (2) by rotary evaporation, 10 times the amount of deionized water is added to precipitate the lignin dissolved in the DES solvent. After vacuum filtration, the filter residue is freeze-dried to obtain lignin powder. The filtrate is evaporated and concentrated to recover the alkaline eutectic solvent.

[0035] (4) The solid component obtained in step (2) is washed three times with deionized water solution, dried and stored in a drying device for later use.

[0036] (5) The subsequent residue was analyzed using the standard method of the National Renewable Energy Laboratory (NREL) in the United States. The results showed that the lignin removal rate was 73.6%, the cellulose retention rate was 89.2%, and the solid residue recovery rate was 68.5%.

[0037] Example 2

[0038] (1) Synthesis of alkaline eutectic solvent: Choline chloride and urea are mixed in a molar ratio of 1:3 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain alkaline eutectic solvent.

[0039] (2) Wheat straw pretreatment: The conditions are similar to those in Example 1, but 3 mL of the alkaline eutectic solvent from step (1) and 2 mL of an aqueous solution containing 20 g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 90 °C and the reaction time is 1 h.

[0040] (3) Subsequent analysis of the residue showed that the lignin removal rate was 71.8%, the cellulose retention rate was 92.7%, and the solid residue recovery rate was 68.2%.

[0041] Example 3

[0042] (1) Synthesis of basic eutectic solvent: choline bromide and ethanolamine are mixed in a molar ratio of 1:5 and stirred continuously at 70°C for 1 hour until the solution becomes transparent to obtain basic eutectic solvent.

[0043] (2) Corn straw pretreatment: The conditions are similar to those in Example 1, but 7 mL of the alkaline eutectic solvent from step (1) and 3 mL of an aqueous solution containing 12 g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 110℃ and the reaction time is 2 h.

[0044] (3) Analysis of the residue showed that the lignin removal rate was 83.5%, the cellulose retention rate was 95.7%, and the solid residue recovery rate was 68.0%.

[0045] Example 4

[0046] (1) Synthesis of basic eutectic solvent: Choline chloride and diethanolamine are mixed in a molar ratio of 1:6 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain basic eutectic solvent.

[0047] (2) Rice straw pretreatment: The conditions are similar to those in Example 1, but 10 mL of the alkaline eutectic solvent from step (1) and 10 mL of an aqueous solution containing 12 g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 60 °C and the reaction time is 3 h.

[0048] (3) Subsequent analysis of the residue showed that the lignin removal rate was 71.2%, the cellulose retention rate was 90.0%, and the solid residue recovery rate was 69.4%.

[0049] Example 5

[0050] (1) Synthesis of basic eutectic solvent: Choline chloride and acetamide are mixed in a molar ratio of 1:6 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain basic eutectic solvent.

[0051] (2) Pretreatment of Miscanthus sinensis: The conditions are similar to those in Example 1, but 3 mL of the alkaline eutectic solvent from step (1) and 7 mL of an aqueous solution containing 12 g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 100℃ and the reaction time is 2 h.

[0052] (3) Analysis of the residue showed that the lignin removal rate was 53.5%, the cellulose retention rate was 90.7%, and the solid residue recovery rate was 78.0%.

[0053] Example 6

[0054] (1) Synthesis of basic eutectic solvent: Acetylcholine chloride and formamide are mixed in a molar ratio of 1:2 and stirred continuously at 90°C for 1 hour until the solution becomes transparent to obtain basic eutectic solvent.

[0055] (2) Rice straw pretreatment: The conditions are similar to those in Example 1, but the amount of straw is 5g and 30mL of the alkaline eutectic solvent from step (1) and 20mL of an aqueous solution containing 12g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 120℃ and the reaction time is 0.5h.

[0056] (3) Subsequent analysis of the residue showed that the lignin removal rate was 76.6%, the cellulose retention rate was 93.2%, and the solid residue recovery rate was 68.2%.

[0057] Example 7

[0058] (1) Corn straw pretreatment: The conditions are similar to those in Example 3, but the alkaline eutectic solvent added is DES recovered after rotary evaporation in Example 3. The reaction temperature is 110℃ and the reaction time is 2h.

[0059] (2) Subsequent analysis of the residue showed that the lignin removal rate was 80.3%, the cellulose retention rate was 92.1%, and the solid residue recovery rate was 69.0%.

[0060] Example 8

[0061] (1) Synthesis of basic eutectic solvent: Choline bromide and acetamide are mixed in a molar ratio of 1:2 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain basic eutectic solvent.

[0062] (2) Rice straw pretreatment: The conditions are similar to those in Example 1, but 10 mL of the alkaline eutectic solvent from step (1) and 10 mL of an aqueous solution containing 9 g / L (NaOH, H2O2, FeSO4·7H2O) are added. The reaction temperature is 100℃ and the reaction time is 2 h.

[0063] (3) Subsequent analysis of the residue showed that the lignin removal rate was 78.0%, the cellulose retention rate was 94.9%, and the solid residue recovery rate was 66.5%.

[0064] Comparative Example 1

[0065] (1) Synthesis of alkaline eutectic solvent: Choline chloride and urea are mixed in a molar ratio of 1:2 and stirred continuously at 80°C for 1 hour until the solution becomes transparent to obtain alkaline eutectic solvent.

[0066] (2) Wheat straw pretreatment: Mix 1g of wheat straw and 10mL of the alkaline eutectic solvent in step (1) evenly, react under normal pressure, react at 90℃ for 1h, cool after the reaction is completed, add 50mL of ethanol to dilute and separate the solid and liquid components, then add an appropriate amount of H2O to wash the reaction solid components. The obtained solid components are wheat straw residue rich in cellulose, and the obtained liquid components are DES components that have dissolved lignin and other degradation products.

[0067] (3) After recovering ethanol from the liquid component obtained in step (2) by rotary evaporation, 10 times the amount of deionized water is added to precipitate the lignin dissolved in the DES solvent. After vacuum filtration, the filter residue is freeze-dried to obtain lignin powder. The filtrate is evaporated and concentrated to obtain a recyclable alkaline eutectic solvent.

[0068] (4) The solid component obtained in step (2) is washed three times with deionized water solution, dried and stored in a drying device for later use.

[0069] (5) Subsequent analysis of the residue showed that the lignin removal rate was 28.3%, the cellulose retention rate was 90.4%, and the solid residue recovery rate was 88.2%. The low lignin removal rate illustrates the importance of the Fenton system's oxidation pretreatment.

[0070] Table 1. Effects of pretreatment on straw under different conditions

[0071]

[0072] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. The application of an alkaline eutectic solvent in the separation of three elements from straw through oxidative pretreatment, characterized in that, The steps include the following: (1) Combine straw, alkaline eutectic solvent and Fenton's reagent (H2O2-Fe) 2+ After mixing NaOH evenly and reacting at 60-120℃ under normal pressure for 0.5-3 hours, add an appropriate amount of ethanol for dilution and filtration, then add water for washing and solid-liquid separation. The obtained solid component is straw residue rich in carbohydrates, and the liquid component is DES component containing lignin and other degradation components. (2) Wash and dry the solid components from step (1) to obtain straw residue; (3) Remove ethanol from the liquid component in step (1), add water to precipitate the lignin dissolved in DES solvent, filter and freeze-dry the filter residue to obtain lignin powder, evaporate and concentrate the filtrate to recover the alkaline eutectic solvent and recycle it. The basic eutectic solvent is prepared by the following method: Using quaternary ammonium salts as hydrogen acceptors and amine / amide compounds as hydrogen donors, the hydrogen acceptors and hydrogen donors are mixed and stirred at 60–100°C for a period of time until the solution becomes clear, thus obtaining an alkaline eutectic solvent. Preferably, the molar ratio of the hydrogen acceptor to the hydrogen donor is 1:2 to 1:

6. Preferably, the quaternary ammonium salt is at least one of choline chloride, choline bromide, and acetylcholine chloride, and the amine / amide group is at least one of urea, formamide, acetamide, diethanolamine, and ethanolamine. The preferred reaction time at 60–100°C is 0.5–2 hours. The bath ratio of straw and alkaline eutectic solvent in step (1) is 1:5 to 1:

20.

2. The application of the alkaline eutectic solvent according to claim 1 in the separation of three elements in the oxidative pretreatment of straw, characterized in that, The concentration range of the Fenton reagent is 6-20 g / L, and the content of H2O is 0-70%.

3. The application of the alkaline eutectic solvent according to claim 1 in the separation of three elements in the oxidative pretreatment of straw, characterized in that, Step (1) The mixing ratio of straw and solvent refers to the mass to the volume ratio of the liquid.

4. The application of the alkaline eutectic solvent according to claim 1 in the separation of three elements in the oxidative pretreatment of straw, characterized in that, The straw mentioned in step (1) is at least one of corn straw, wheat straw, rice straw and miscanthus.

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