Method for preparing bio-based polyols by microwave treatment of lignocellulose directed depolymerization

Abstract

The present application belongs to the technical field of high-value utilization of biomass, and discloses a method for preparing bio-based polyols by microwave treatment of lignocellulose directional depolymerization, which comprises the following steps: crushing lignocellulose raw materials composed of pinus sylvestris, Chinese fir, bagasse, corn straw, poplar and bamboo chips to obtain lignocellulose powder; adjusting the lignocellulose powder by spraying deionized water to obtain powder and placing the powder in a closed condition for static balance; then uniformly mixing the powder and deionized water to obtain a mixture, and sequentially performing ultrasonic treatment and low-temperature microwave pretreatment on the mixture; then adding a catalyst to the mixture, mixing again, and then performing high-temperature microwave treatment on the mixture in three stages to achieve directional depolymerization; cooling and filtering the treated material to obtain a filtrate; and performing low-temperature concentration on the filtrate to obtain high-purity bio-based mixed polyols. The present application realizes directional depolymerization and activation modification of lignocellulose by ultrasonic pretreatment combined with segmented variable-frequency microwave treatment and catalysis, and is suitable for continuous production of high-activity bio-based polyols.

Description

Technical Field

[0001] This invention belongs to the field of biomass high-value utilization technology, specifically relating to a method for preparing bio-based polyols by microwave treatment of lignocellulose for directional depolymerization. Background Technology

[0002] Lignocellulose, primarily composed of cellulose, hemicellulose, and lignin, is widely found in agricultural and forestry wastes such as wood, straw, bagasse, and sawdust. Its efficient depolymerization and activation into high-value-added platform compounds (such as polyols, phenols, and monosaccharides) is a key pathway to replace petrochemical raw materials. Existing lignocellulose treatment and depolymerization technologies have significant shortcomings. Traditional processes such as hydrothermal, acid hydrolysis, and alkaline boiling are energy-intensive, time-consuming, and generate large amounts of wastewater and waste liquid, causing significant environmental pollution. Furthermore, direct microwave heating easily leads to localized overheating, excessive carbonization of components, and uncontrollable product development, resulting in generally low yields. Moreover, lignin is tightly bound to cellulose and hemicellulose, making it difficult for microwaves to disrupt their dense structure, resulting in incomplete depolymerization. The depolymerization process also lacks selectivity, with simultaneous and disordered degradation of the three components, leading to complex and difficult-to-separate products. The treated products have low activity and insufficient hydroxyl values, making them unsuitable for direct use in high-value fields such as polyurethane and adhesives. In addition, most processes require high pressure, organic solvents, and expensive catalysts, resulting in large equipment investments and hindering their adoption by small and medium-sized enterprises. Therefore, there is still a need to develop a method for the continuous processing of lignocellulose in a targeted, controllable, efficient, and low-cost manner to prepare bio-based polyols, thereby realizing the high-value utilization of lignocellulose. Summary of the Invention

[0003] To address the aforementioned shortcomings, this invention discloses a method for preparing bio-based polyols by microwave treatment of lignocellulose through directional depolymerization. The method utilizes ultrasonic pretreatment combined with segmented frequency conversion microwave treatment to synergistically catalyze the directional depolymerization and activation modification of lignocellulose, making it suitable for continuous production of highly active bio-based polyols.

[0004] This invention is achieved using the following technical solution: A method for preparing bio-based polyols by microwave treatment of lignocellulose for directional depolymerization includes the following steps: (1) Take lignocellulose raw materials and crush them to obtain lignocellulose powder. The lignocellulose raw materials include one or more combinations of eucalyptus, pine, fir, bagasse, corn stalks, poplar and bamboo chips. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 12-22%. Place the powder in a sealed environment and let it stand for 2 hours to allow the moisture to penetrate evenly into the intercellular gaps, thereby improving the uniformity of the wood powder and also enhancing the uniformity of the ultrasonic and microwave response. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g: (3-6)mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency of 20-40KHz, ultrasonic power of 200-500W, and ultrasonic treatment time of 10-30min. The cell wall is broken by ultrasonic treatment, and the cell wall is torn by cavitation effect, the lignin and cellulose are separated, the fiber structure is loosened and modified, and the subsequent microwave depolymerization activation energy is reduced. (3) Take the mixture processed in step (2) and perform microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency is 915MHz, microwave power is 300W, microwave treatment time is 30-40s, material temperature is controlled at 70-95℃, microwave preheating is performed in advance to further loosen the cellulose crystal region and fully expand the cell wall structure, thereby reducing the subsequent depolymerization resistance. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage treatment are a microwave frequency of 2450MHz and an energy density of 10-30kWh / m³. 3 The first stage of treatment involves a processing time of 20–40 seconds and a material temperature of 70–95°C. The second stage of treatment requires a microwave frequency of 2450 MHz and an energy density of 35–60 kWh / m³. 3 The processing time is 30–60 seconds, and the material temperature is 110–145℃. The conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 65–90 kWh / m³. 3 The processing time is 40–80 seconds, and the material temperature is 150–190°C. (5) Take the mixture treated in step (4) and cool it to below 40°C. Then filter it to obtain filtrate. Then concentrate the filtrate at low temperature to obtain high-purity bio-based mixed polyol. The solid residue obtained after the filtrate is concentrated at low temperature is a high-porosity cellulose material, which can be directly used for adsorption, filling, and artificial board substrate.

[0005] This invention achieves the directional conversion of lignocellulose through ultrasonic pretreatment combined with segmented frequency-converting microwave chemical depolymerization. In the ultrasonic treatment stage, cavitation effect is used to break the dense structure, strip lignin, and open cell wall channels. Then, in the low-frequency microwave band, cellulose hydrogen bonds are broken, crystalline regions are loosened, and cell walls are expanded. Next, in the medium-frequency microwave band, hemicellulose is selectively degraded into small-molecule sugar alcohols. Finally, in the high-frequency microwave band, lignin β-O-4 bonds are directionally broken to generate high-phenolic hydroxyl products. Throughout the microwave degradation process, a solid acid-base catalyst composed of sodium carbonate and sodium hydroxide is added, which stabilizes intermediate products, prevents condensation / carbonization, and facilitates the one-step acquisition of high-purity polyols. Ultimately, this achieves cellulose retention and activation, efficient hemicellulose degradation, directional bond breaking of lignin, and the stepwise conversion and full utilization of cellulose, hemicellulose, and lignin.

[0006] Furthermore, in step (1), the lignocellulose raw material is crushed and passed through a 10-60 mesh sieve to obtain lignocellulose powder.

[0007] Furthermore, in step (2), the material temperature is controlled to not exceed 50°C during the ultrasonic treatment process.

[0008] Furthermore, in step (3), the mixture treated in step (2) is filtered and dehydrated to a moisture content of 15-25% and then subjected to microwave pretreatment.

[0009] Furthermore, in step (4), the amount of catalyst used is 0.5% to 3% of the mass of the mixture. The catalyst is obtained by mixing sodium carbonate and sodium hydroxide, and the mass ratio of sodium carbonate to sodium hydroxide is (1-2):(3-5). By adding the composite catalyst of sodium carbonate and sodium hydroxide, the degradation products are hydrogenated, reformed, and stabilized in situ, directly converted into stable and highly active polyols, and the efficient degradation of lignocellulose is promoted.

[0010] Furthermore, in step (4), the microwave is output in a pulsed intermittent manner during the third stage of processing.

[0011] Furthermore, in step (5), the mixture is cooled to below 40°C at a rate of 10-30°C / min.

[0012] Compared with existing technologies, this technical solution has the following advantages: 1. This invention first employs ultrasonic treatment of lignocellulose powder. Ultrasonic waves can break the dense structure of lignocellulose through cavitation and mechanical vibration, tearing cell walls and stripping surface lignin, significantly reducing resistance to subsequent processing. Simultaneously, ultrasonic waves do not damage the chemical components, only physically breaking the dense structure and stripping lignin, allowing microwave energy to penetrate more evenly into the interior, avoiding carbonization and improving depolymerization efficiency. Next, microwave treatment is used to directionally depolymerize the lignocellulose powder. Microwaves, as high-frequency electromagnetic waves, have advantages such as selective heating, internal heating, fast reaction speed, and being green and pollution-free. Through the combined application of ultrasound and microwaves, different components can be precisely targeted, achieving cellulose expansion, selective degradation of hemicellulose, and directional bond breaking of lignin, without requiring large amounts of solvents, high pressure, or wastewater discharge, making it green and efficient.

[0013] 2. The present invention performs microwave treatment in three stages. The first stage is a low-frequency expansion stage, in which the cellulose crystal regions are loosened and the cell walls are fully expanded. The second stage is a medium-frequency selective depolymerization stage, in which hemicellulose is efficiently degraded into pentose sugars and small molecule alcohols. The third stage is a high-frequency directional bond breaking stage, in which the β-O-4 bonds of lignin are directionally broken to generate phenolic polyols.

[0014] 3. The polyol product prepared by this invention has high activity and good stability, and can be directly used in polyurethane and formaldehyde-free adhesives. Furthermore, the solid residue generated during the production process can be directly used as an adsorbent, filler, and substrate for engineered wood products, resulting in high resource utilization. In addition, the method described in this invention is simple to operate, highly controllable, and adaptable to industrial, large-scale, and continuous production lines, which is beneficial for the centralized treatment of agricultural and forestry waste. Detailed Implementation

[0015] The present invention is further illustrated by the following examples, but these are not intended to limit the invention. Specific experimental conditions and methods not specified in the following examples are generally conventional methods well known to those skilled in the art.

[0016] Example 1: A method for preparing bio-based polyols by microwave-treated lignocellulose through directional depolymerization, characterized by the following steps: (1) Take the lignocellulose raw material and crush it through a 40-mesh sieve to obtain lignocellulose powder. The lignocellulose raw material is a combination of eucalyptus wood, bagasse and corn stalks. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 18%. Place the powder under sealed conditions and let it stand for 2 hours to reach equilibrium. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g:4mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency is 30KHz, ultrasonic power is 300W, ultrasonic treatment time is 20min, and the material temperature is controlled not to exceed 50℃ during the ultrasonic treatment. (3) After the mixture treated in step (2) is filtered and dehydrated to a moisture content of 20%, it is subjected to microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency of 915MHz, microwave power of 300W, microwave treatment time of 30s, and material temperature controlled at 80℃. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage of treatment are a microwave frequency of 2450MHz and an energy density of 2kWh / m³. 3 The first stage of treatment has a processing time of 30 seconds and a material temperature of 90°C. The second stage of treatment has a microwave frequency of 2450MHz and an energy density of 60kWh / m³. 3 The processing time is 60 seconds, the material temperature is 135℃, and the conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 90kWh / m³. 3 The processing time is 60 seconds, the material temperature is 180℃, and the microwave uses a pulse intermittent output mode. The catalyst is used at 1.5% of the mass of the mixture, and the catalyst is obtained by mixing sodium carbonate and sodium hydroxide, with a mass ratio of sodium carbonate to sodium hydroxide of 1.5:4. (5) Take the mixture treated in step (4) and cool it down to 35°C at a rate of 20°C / min. Then filter it to obtain filtrate. Then concentrate the filtrate at low temperature to obtain high-purity bio-based mixed polyol.

[0017] Example 2: A method for preparing bio-based polyols by microwave-treated lignocellulose through directional depolymerization, characterized by the following steps: (1) Take the lignocellulose raw material and crush it through a 10-mesh sieve to obtain lignocellulose powder. The lignocellulose raw material is a combination of eucalyptus, fir, bagasse and bamboo chips. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 12%. Place the powder under sealed conditions and let it stand for 2 hours to reach equilibrium. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g:6mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency is 20KHz, ultrasonic power is 200W, ultrasonic treatment time is 30min, and the material temperature is controlled not to exceed 50℃ during the ultrasonic treatment. (3) After the mixture treated in step (2) is filtered and dehydrated to a moisture content of 15%, it is subjected to microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency of 915MHz, microwave power of 300W, microwave treatment time of 35s, and material temperature controlled at 70℃. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage treatment are a microwave frequency of 2450MHz and an energy density of 10kWh / m³. 3 The first stage of treatment involves a processing time of 40 seconds and a material temperature of 70°C. The second stage of treatment is characterized by a microwave frequency of 2450MHz and an energy density of 35kWh / m³. 3 The processing time is 50 seconds, the material temperature is 110℃, and the conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 65kWh / m³. 3 The processing time is 80 seconds, the material temperature is 150°C, and the microwave uses a pulse intermittent output mode. The amount of catalyst used is 0.5% of the mass of the mixture. The catalyst is obtained by mixing sodium carbonate and sodium hydroxide, and the mass ratio of sodium carbonate to sodium hydroxide is 1:3. (5) Take the mixture treated in step (4) and cool it down to 40°C at a rate of 10°C / min. Then filter it to obtain filtrate. Then concentrate the filtrate at low temperature to obtain high-purity bio-based mixed polyol.

[0018] Example 3: A method for preparing bio-based polyols by microwave treatment of lignocellulose through directional depolymerization, characterized by the following steps: (1) Take the lignocellulose raw material and crush it through a 30-mesh sieve to obtain lignocellulose powder. The lignocellulose raw material is a combination of eucalyptus, pine and bamboo chips. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 20%. Place the powder under sealed conditions and let it stand for 2 hours to reach equilibrium. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g:5mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency is 30KHz, ultrasonic power is 400W, ultrasonic treatment time is 15min, and the material temperature is controlled not to exceed 50℃ during the ultrasonic treatment. (3) After the mixture treated in step (2) is filtered and dehydrated to a moisture content of 20%, it is subjected to microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency of 915MHz, microwave power of 300W, microwave treatment time of 35s, and material temperature controlled at 85℃. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage of treatment are a microwave frequency of 2450MHz and an energy density of 25kWh / m³. 3 The first stage of treatment involved a processing time of 35 seconds and a material temperature of 90°C. The second stage of treatment was conducted under the following conditions: a microwave frequency of 2450 MHz and an energy density of 50 kWh / m³. 3 The processing time is 50 seconds, the material temperature is 130℃, and the conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 80kWh / m³. 3 The processing time is 50 seconds, the material temperature is 160°C, and the microwave uses a pulse intermittent output mode. The catalyst is used at 2% of the mass of the mixture, and the catalyst is obtained by mixing sodium carbonate and sodium hydroxide, with a mass ratio of sodium carbonate to sodium hydroxide of 1.5:4.5. (5) Take the mixture treated in step (4) and cool it down to 38°C at a rate of 25°C / min. Then filter it to obtain the filtrate. Then concentrate the filtrate at low temperature to obtain a high-purity bio-based mixed polyol.

[0019] Example 4: A method for preparing bio-based polyols by microwave-treated lignocellulose through directional depolymerization, characterized by the following steps: (1) Take the lignocellulose raw material and crush it through a 60-mesh sieve to obtain lignocellulose powder. The lignocellulose raw material is a combination of eucalyptus, pine and fir. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 22%. Place the powder under sealed conditions and let it stand for 2 hours to reach equilibrium. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g:3mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency is 40KHz, ultrasonic power is 500W, ultrasonic treatment time is 10min, and the material temperature is controlled not to exceed 50℃ during the ultrasonic treatment. (3) After the mixture treated in step (2) is filtered and dehydrated to a moisture content of 25%, it is subjected to microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency of 915MHz, microwave power of 300W, microwave treatment time of 40s, and material temperature controlled at 95℃. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage treatment are a microwave frequency of 2450MHz and an energy density of 30kWh / m³. 3The first stage of treatment involves a processing time of 20 seconds and a material temperature of 95°C. The second stage of treatment is characterized by a microwave frequency of 2450MHz and an energy density of 60kWh / m³. 3 The processing time is 30 seconds, the material temperature is 145℃, and the conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 90kWh / m³. 3 The processing time is 40 seconds, the material temperature is 190℃, and the microwave uses a pulse intermittent output mode. The catalyst is used at 3% of the mass of the mixture, and the catalyst is obtained by mixing sodium carbonate and sodium hydroxide, with a mass ratio of sodium carbonate to sodium hydroxide of 2:5. (5) Take the mixture treated in step (4) and cool it down to 38°C at a rate of 30°C / min. Then filter it to obtain filtrate. Then concentrate the filtrate at low temperature to obtain high-purity bio-based mixed polyol.

[0020] Comparative Example 1: The method for preparing bio-based polyols by microwave treatment of lignocellulose in this comparative example differs from that in Example 1 only in that the ultrasonic treatment step is omitted, and the powder treated in step (1) is directly subjected to microwave pretreatment, while the other steps remain unchanged.

[0021] Comparative Example 2: The method for preparing bio-based polyols by microwave treatment of lignocellulose in this comparative example differs from that in Example 1 only in that, in step (4), the mixture treated in step (3) is added to the catalyst and mixed evenly, and then microwave treatment is performed under the following conditions: microwave frequency of 2450MHz and energy density of 60kWh / m³. 3 The processing time is 120 seconds, the material temperature is 120°C, and the other steps remain unchanged.

[0022] Comparative Example 3: The method for preparing bio-based polyols by microwave treatment of lignocellulose in this comparative example differs from that in Example 1 only in that, in step (4), the mixture treated in step (3) is added to the catalyst and mixed evenly, and then microwave treatment is performed under the following conditions: microwave frequency of 915MHz and energy density of 90kWh / m³. 3 The processing time is 120 seconds, the material temperature is 150°C, and the other steps remain unchanged.

[0023] Experimental Example: The polyol products prepared according to the methods described in Examples 1-4 and Comparative Examples 1-3 were tested, and the specific results are shown in Table 1. According to the test results in Table 1, the polyol products obtained by the method of this invention have higher hydroxyl values ​​and lower water content and ash content, indicating that this invention can obtain polyol products with high stability and high activity, and the yield exceeds 75%. In contrast, Comparative Example 1 did not use ultrasonic pretreatment, which is detrimental to subsequent microwave degradation, resulting in a significant decrease in product yield. Furthermore, Comparative Examples 2 and 3 did not perform staged microwave treatment but used uniform microwave conditions, resulting in lower product yields than the method described in this invention.

[0024] Table 1. Detection results of polyol products prepared by different methods

[0025] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization, characterized in that: Includes the following steps: (1) Take lignocellulose raw materials and crush them to obtain lignocellulose powder. The lignocellulose raw materials include one or more combinations of eucalyptus, pine, fir, bagasse, corn stalks, poplar and bamboo chips. Spray deionized water into the lignocellulose powder to obtain powder with a moisture content of 12-22%. Place the powder in a sealed environment and let it stand for 2 hours to reach equilibrium. (2) Take the powder obtained in step (1) and deionized water and mix them evenly to obtain a mixture. The ratio of the amount of powder to deionized water is 1g: (3-6)mL. The mixture is subjected to ultrasonic treatment and the ultrasonic treatment conditions are: ultrasonic frequency of 20-40KHz, ultrasonic power of 200-500W, and ultrasonic treatment time of 10-30min. (3) Take the mixture processed in step (2) and perform microwave pretreatment. The conditions for microwave pretreatment are: microwave frequency is 915MHz, microwave power is 300W, microwave treatment time is 30-40s, and material temperature is controlled at 70-95℃. (4) Take the mixture treated in step (3), add the catalyst and mix evenly. Then, perform microwave treatment in three stages. The conditions for the first stage treatment are a microwave frequency of 2450MHz and an energy density of 10-30kWh / m³. 3 The first stage of treatment involves a processing time of 20–40 seconds and a material temperature of 70–95°C. The second stage of treatment requires a microwave frequency of 2450 MHz and an energy density of 35–60 kWh / m³. 3 The processing time is 30–60 seconds, and the material temperature is 110–145℃. The conditions for the third stage of processing are a microwave frequency of 2450MHz and an energy density of 65–90 kWh / m³. 3 The processing time is 40–80 seconds, and the material temperature is 150–190°C. (5) Take the mixture processed in step (4) and cool it to below 40°C. Then filter it to obtain filtrate. Then concentrate the filtrate at low temperature to obtain high-purity bio-based mixed polyol.

2. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (1), the lignocellulose raw material is crushed and passed through a 10-60 mesh sieve to obtain lignocellulose powder.

3. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (2), the material temperature is controlled to not exceed 50°C during the ultrasonic treatment process.

4. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (3), the mixture treated in step (2) is filtered and dehydrated to a moisture content of 15-25% and then subjected to microwave pretreatment.

5. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (4), the amount of catalyst used is 0.5% to 3% of the mass of the mixture. The catalyst is obtained by mixing sodium carbonate and sodium hydroxide, and the mass ratio of sodium carbonate to sodium hydroxide is (1 to 2): (3 to 5).

6. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (4), the microwave is output in a pulsed intermittent manner during the third stage of processing.

7. The method for preparing bio-based polyols by microwave treatment of lignocellulose with directional depolymerization according to claim 1, characterized in that: In step (5), the mixture is cooled to below 40°C at a rate of 10-30°C / min.

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