A method for preparing hydrophobic silica aerogel for cryogenic environment

Abstract

The application discloses a preparation method of hydrophobic silica aerogel for low-temperature environment, and relates to the technical field of aerogel preparation.The method uses agricultural waste as a silicon source, directly fills a wet gel into a kettle, and omits a solvent exchange step.By constructing a solvent-free system with supercritical carbon dioxide as a medium, adding a lignin derivative and a low-temperature enzyme catalyst, and synchronously completing water replacement, enzyme catalytic hydrophobic modification and drying in a supercritical state, the carbon dioxide is recovered when pressure is released, and finally, the hydrophobic silica aerogel product suitable for the low-temperature environment is obtained.The application realizes high-value utilization of resources and cost reduction, constructs a solvent-free homogeneous phase reaction system, realizes cyclic utilization of biomass resources and green preparation, and simultaneously, through integrated supercritical processing and recovery of the carbon dioxide medium, the product is endowed with excellent hydrophobicity, mechanical strength and low-temperature stability on the basis of reservation of the three-dimensional network structure of the aerogel, and the application prospect of the product in the low-temperature environment is expanded.

Description

Technical Field

[0001] This invention relates to the field of aerogel preparation technology, specifically to a method for preparing hydrophobic silica aerogels for low-temperature environments. Background Technology

[0002] Silica aerogel, as a novel nanoporous thermal insulation material, has demonstrated high application value in low-temperature environment-related applications due to its extremely low thermal conductivity, excellent hydrophobic properties, and lightweight porous structure, becoming one of the current research hotspots in the materials field. With the rapid development of industries such as cold chain logistics, ultra-low temperature storage, aerospace, and new energy cryogenic equipment, the market has placed higher demands on thermal insulation materials suitable for low-temperature environments. These materials not only need stable hydrophobic properties and mechanical strength but also require stable performance under ultra-low temperature conditions. Meanwhile, the green, environmentally friendly, and resource-sustainable preparation concept has become an important guide for industry development. Agricultural waste, as an important component of biomass resources, is crucial for solving agricultural solid waste pollution and achieving resource recycling. The application of supercritical fluid technology in the field of materials preparation provides technical support for abandoning traditional organic solvents and achieving green preparation. Against this backdrop, developing a preparation process for hydrophobic silica aerogels for low-temperature environments based on biomass resources has significant practical implications.

[0003] Traditional processes for preparing hydrophobic silica aerogels face numerous challenges in practical implementation. Firstly, the silicon source often relies on chemically synthesized raw materials, resulting in high production costs and failing to align with sustainable resource development needs. While some processes attempt to extract silicon from biomass, the extraction efficiency is low and the silicon source lacks sufficient activity, impacting subsequent gel preparation. Secondly, traditional processes typically include a solvent exchange step, requiring large amounts of organic solvents, increasing environmental impact, process complexity, and energy consumption. The hydrophobic modification stage often relies on synthetic modifiers, with demanding reaction conditions and separate steps from drying. This multi-step process not only reduces overall preparation efficiency but also risks damaging the gel's three-dimensional network structure. Furthermore, traditional drying processes often lead to insufficient mechanical strength and poor bonding stability of the hydrophobic modification, causing performance degradation and structural deformation in ultra-low temperature environments, failing to meet long-term low-temperature requirements. Additionally, some processes fail to effectively recycle the reaction medium, wasting resources and further increasing environmental costs. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing hydrophobic silica aerogel for low-temperature environments. This method uses agricultural waste as the silicon source, prepares wet gel, and directly loads it into a supercritical reactor. By constructing a solvent-free system with supercritical carbon dioxide as the medium, and introducing activated lignin derivatives and low-temperature enzyme catalysts, water replacement, enzyme-catalyzed hydrophobic modification, and supercritical drying are completed simultaneously through supercritical integrated treatment. Finally, carbon dioxide is recovered by depressurization, and the product is tested to obtain the biomass-reinforced hydrophobic silica aerogel.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing hydrophobic silica aerogel for low-temperature environments, the specific steps of which are as follows: S100, Wet Gel Preparation and Loading: Active silicon source is extracted from agricultural waste as raw material, and highly active silica sol is obtained through alkali dissolution, acidification, aging and washing. Silica wet gel is prepared by sol-gel method. The prepared wet gel is directly loaded into supercritical reactor. No solvent exchange operation between organic solvent and water is performed throughout the process. S200, Solvent-free system construction: Liquid CO2 is introduced into a supercritical reactor containing silica wet gel, and purified and activated lignin derivatives and low-temperature enzyme catalysts are added at the same time. Stirring is turned on to make the materials uniformly dispersed in CO2, thus constructing a solvent-free homogeneous reaction system with supercritical CO2 as the medium. S300, supercritical integrated treatment: Adjust the process parameters of the supercritical reactor to 31℃ and 7.39MPa, maintain the supercritical state of CO2 for 6-8 hours, and simultaneously complete the water replacement of wet gel, enzyme-catalyzed hydrophobic modification and supercritical fluid drying. S400, depressurization and CO2 recovery: After the supercritical integrated treatment is completed, the pressure inside the reactor is slowly reduced to atmospheric pressure at a constant rate of 0.1 to 0.3 MPa / min. During the depressurization process, the purified CO2 discharged is recovered simultaneously. S500 Finished Product Discharge Inspection: After the reactor is completely depressurized to room temperature and pressure, the product inside the reactor is taken out and tested for properties such as water contact angle, compressive strength, and ultra-low temperature stability. After the performance test, a biomass-reinforced hydrophobic silica aerogel product suitable for low temperature environment is obtained.

[0006] Furthermore, the agricultural waste is one or more of rice straw, wheat straw, and corn straw. The agricultural waste is incinerated in an air atmosphere to obtain straw ash. The incineration process parameters are: incineration temperature of 500℃~700℃, incineration time of 2 hours~4 hours, and the straw ash obtained after incineration is crushed and passed through an 80-120 mesh sieve. The mass fraction of silicon dioxide in the straw ash after sieving is not less than 85%.

[0007] Furthermore, the extraction of the active silicon source requires mixing straw ash with a 10%–20% sodium hydroxide solution at a solid-liquid ratio of 1:5–1:10, and stirring the mixture at a rate of 300–500 r / min for 3–5 hours in a water bath at 80–95°C. After the reaction is complete, the mixture is filtered through a 100–200 mesh screen to remove insoluble carbon residue and impurities, yielding a sodium silicate solution. A 5%–15% dilute sulfuric acid solution is then added dropwise to the sodium silicate solution to adjust the pH to 3–5, causing silicon dioxide to precipitate as hydrated silicic acid. After precipitation, the solution is allowed to stand and age at 20–30°C for 12–24 hours. After aging, the solution is repeatedly washed with deionized water until the conductivity of the filtrate is below 100 μS / cm, yielding a highly active silica sol with a silicon dioxide mass fraction of 10%–20%.

[0008] Furthermore, the process for preparing silica wet gel by the sol-gel method involves adding 0.5%–2% ammonia water by mass to a highly active silica sol as a gel catalyst, adjusting the pH of the silica sol to 8–10, and allowing it to gel at a constant temperature of 25°C–35°C for 1–3 hours. After the silica sol has completely formed a transparent gel, it is aged in a constant temperature oven at 40°C–60°C for 24–48 hours to obtain a silica wet gel with a continuous three-dimensional network structure. The solid content of the wet gel is 5%–15%, and the porosity of the wet gel is not less than 90%.

[0009] Furthermore, the lignin derivative is one or more combinations of enzymatically hydrolyzed lignin, alkali lignin, and sulfonated lignin. The lignin derivative is purified and activated by reflux washing the lignin raw material with a 95% ethanol solution at 50℃~60℃ 3 to 5 times, with each washing time lasting 1h~2h, to remove residual sugars, hemicellulose, and ash impurities from the raw material. After washing, the raw material is dried in an oven at 105℃~110℃ for 2h~4h, pulverized, and passed through an 80-100 mesh sieve to obtain the activated lignin derivative. The amount of lignin derivative added is 5%~10% of the mass of CO2 introduced.

[0010] Furthermore, the low-temperature enzyme catalyst is a low-temperature lipase, the optimal reaction temperature of which is 0℃~35℃, the enzyme activity is not less than 10000U / g, and the addition amount is 0.5%~2% of the mass of the silica wet gel. The introduced CO2 is food-grade liquid CO2 with a purity of not less than 99.9%. The initial temperature in the supercritical reactor when introducing CO2 is 10℃~20℃ and the initial pressure is 3MPa~5MPa. During the introduction process, the stirring device of the reactor is turned on, the stirring rate is 200r / min~400r / min, and the stirring time is 30min~60min, forming a homogeneous reaction system that is completely free of organic solvents and synthetic modifiers.

[0011] Furthermore, the supercritical reactor is made of stainless steel, with a temperature control accuracy of ±0.5℃ and a pressure control accuracy of ±0.05MPa. When adjusting the process parameters, the temperature is first increased to 31℃ at a rate of 1℃ / min to 2℃ / min, and then the pressure is increased to 7.39MPa at a rate of 0.5MPa / min to 1MPa / min. After the temperature and pressure inside the reactor stabilize, the supercritical state is maintained for 6h to 8h. During the heat preservation process, the stirring device is turned on every 1h to 2h, and the stirring is carried out at a rate of 150r / min to 300r / min for 10min to 15min.

[0012] Furthermore, the wet gel water replacement involves permeating supercritical CO2 into the three-dimensional network pores of the silica wet gel to replace the water inside the wet gel; enzyme-catalyzed hydrophobic modification involves lignin derivatives undergoing a bonding reaction with the silanol groups on the silica surface under the action of low-temperature enzymes; and supercritical fluid drying involves drying the gel with supercritical CO2 under conditions of no surface tension, preserving the three-dimensional network structure of silica.

[0013] Furthermore, the pressure relief process employs a multi-stage throttling pressure relief device. During the pressure relief process, the temperature inside the reactor is kept stable at 30℃~32℃, and the pressure relief rate is strictly controlled at 0.1MPa / min~0.3MPa / min until the pressure inside the reactor drops to atmospheric pressure. CO2 recovery involves condensing the gaseous CO2 discharged from the pressure relief process into liquid state through a condenser at -10℃ to 0℃ and 2MPa~3MPa. Then, impurities are removed by an activated carbon adsorption tower and a molecular sieve purification tower. The purity of the purified CO2 is not less than 99.9%, and the CO2 recovery rate is not less than 95%.

[0014] Furthermore, the performance tests include water contact angle testing, compressive strength testing, and cryogenic stability testing. The water contact angle test uses the seated drop method with deionized water as the test solution at 25°C, and the initial water contact angle of the aerogel is not less than 140°. The compressive strength test uses a universal testing machine with a sample size of 20mm×20mm×20mm and a testing rate of 1mm / min, and the compressive strength of the aerogel is not less than 1.7MPa. The cryogenic stability test involves placing the aerogel in a -120°C cryogenic chamber for 1000 hours continuously, with a temperature fluctuation range of ±2°C. During the test, the sample is not subjected to any external load. After the test, the water contact angle of the aerogel is not less than 135°, the compressive strength retention rate is not less than 90%, and the sample shows no obvious cracking or deformation.

[0015] Compared with existing technologies, this method for preparing hydrophobic silica aerogels for low-temperature environments has the following advantages: I. This invention extracts active silicon sources from agricultural waste, realizing the high-value recycling of agricultural biomass resources. This effectively mitigates the environmental problems caused by agricultural waste and reduces the raw material cost of aerogel preparation, aligning with the green and low-carbon development concept. After preparing silica wet gel, the solvent exchange operation is eliminated throughout the process, greatly simplifying the aerogel preparation process and effectively shortening the overall production cycle. Liquid CO2 is introduced into the supercritical reactor, and a solvent-free homogeneous reaction system is constructed by combining purified and activated lignin derivatives with a low-temperature enzyme catalyst. This eliminates the use of organic solvents and synthetic modifiers, solving the pollution problem of chemical reagents at the source. At the same time, the lignin derivatives enhance the aerogel by modifying it with biomass, improving the basic stability of the aerogel from a structural level, making the entire preparation process both economical and environmentally friendly.

[0016] II. This invention achieves simultaneous completion of wet gel water replacement, enzyme-catalyzed hydrophobic modification, and supercritical fluid drying by controlling the process conditions of a supercritical reactor. It integrates multiple processes into a single operation, improving the production efficiency of aerogels. Relying on the medium characteristics of supercritical CO2, the continuous three-dimensional network structure of silica aerogels is completely preserved during the processing, ensuring the porosity of the aerogel core. During the depressurization stage, the pressure is slowly reduced at a constant rate while the purified CO2 is simultaneously recovered, realizing the recycling of the medium and further reducing energy consumption and material costs in the production process. The prepared aerogels combine the dual advantages of hydrophobic modification and biomass enhancement, possessing excellent hydrophobic properties, mechanical strength, and ultra-low temperature stability. They can maintain structural and performance stability for a long time in low-temperature environments, effectively expanding the application fields and scenarios of silica aerogels.

[0017] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0019] Figure 1 This is a framework diagram of a method for preparing hydrophobic silica aerogels for low-temperature environments. Figure 2 This is a flowchart of a method for preparing hydrophobic silica aerogel for low-temperature environments. Detailed Implementation

[0020] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below. Example

[0021] Wet gel preparation: Rice straw was selected as the agricultural waste raw material. After impurity removal and chopping, the rice straw was placed in a muffle furnace and incinerated in air. The incineration temperature was controlled at 500℃ for 4 hours. After incineration, it was naturally cooled to room temperature to obtain straw ash. The straw ash was then pulverized in a universal pulverizer. The pulverized material was passed through an 80-mesh sieve according to standard inspection. The material passing through the sieve was qualified straw ash. The mass fraction of silica in the straw ash was determined to be 86% by gravimetric analysis.

[0022] Take the above-mentioned qualified straw ash and mix it with a 10% sodium hydroxide solution at a solid-liquid ratio of 1:5. Transfer the mixture into a reaction vessel with a constant temperature water bath stirrer. Set the water bath temperature to 80℃ and the stirring rate to 300 r / min, and continue stirring for 5 hours. After the reaction is complete, filter the material through a 100-mesh filter to remove insoluble carbon residue and impurities from the filtrate, obtaining a clear sodium silicate solution. Add a 5% dilute sulfuric acid solution dropwise to the sodium silicate solution through a constant pressure dropping funnel, stirring continuously during the addition process, until the pH value of the solution is adjusted to 3. At this point, silica precipitates from the solution in the form of hydrated silicic acid. Place the precipitate in a constant temperature incubator and let it stand and age at a constant temperature of 20℃ for 24 hours.

[0023] After aging, the material was transferred into a Buchner funnel for filtration. The filtered solid material was then repeatedly washed with deionized water. After each wash, the conductivity of the filtrate was measured with a conductivity meter until the conductivity of the filtrate was lower than 100 μS / cm. Washing was then stopped and the material was dispersed in deionized water to obtain a highly active silica sol. The mass fraction of silica in the silica sol was determined to be 10% by gravimetric analysis.

[0024] Ammonia solution with a mass fraction of 0.5% was added dropwise to the highly active silica sol as a gelation catalyst, with continuous stirring during the addition process, until the pH of the silica sol was adjusted to 8. The adjusted silica sol was then placed in a constant temperature incubator and allowed to gel at 25°C for 3 hours. After the silica sol had completely formed a transparent gel, it was transferred to an electrically heated constant temperature oven and aged at 40°C for 48 hours to obtain a silica wet gel with a continuous three-dimensional network structure. The solid content of the wet gel was found to be 5%, and the porosity was 91%. The prepared silica wet gel was then completely transferred into the reaction chamber of a supercritical reactor, ensuring that the three-dimensional network structure of the wet gel remained intact during the loading process, without any solvent exchange. Figure 1 As shown.

[0025] Construction of solvent-free system: Enzymatic hydrolyzed lignin was selected as the raw material for lignin derivatives. It was purified and activated. The enzymatic hydrolyzed lignin raw material was placed in a Soxhlet extractor, and a 95% ethanol solution was added. The water bath temperature was set to 50℃, and the material was refluxed and washed 5 times, with each washing time being 1 hour. After washing, the material was taken out and placed in an electric constant temperature oven and dried at 105℃ for 4 hours. The dried material was then pulverized by a universal pulverizer and passed through an 80-mesh sieve to obtain activated enzymatic hydrolyzed lignin derivatives.

[0026] Food-grade liquid CO2 was introduced into a supercritical reactor containing silica wet gel. Before introduction, the initial temperature of the reactor was adjusted to 10°C and the initial pressure to 3 MPa. The purity of the liquid CO2 was tested to be no less than 99.9%. Subsequently, the above-mentioned activated lignin-hydrolyzing derivative and low-temperature lipase were added to the reactor. The amount of activated lignin-hydrolyzing derivative added was 5% of the mass of CO2 introduced. The low-temperature lipase was a commercially available qualified product with an optimal reaction temperature of 0°C to 35°C and an enzyme activity of 10,000 U / g. The amount added was 0.5% of the mass of silica wet gel.

[0027] Turn on the paddle mechanical stirrer of the reactor, set the stirring speed to 200 r / min, and stir continuously for 60 min to uniformly disperse the activated enzymatic hydrolyzed lignin derivative, low-temperature lipase and silica wet gel in liquid CO2, forming a homogeneous reaction system completely free of organic solvents and synthetic modifiers, thus completing the construction of the solvent-free system.

[0028] Supercritical Integrated Processing: The supercritical reactor used in this embodiment is a stainless steel high-pressure reactor equipped with a precise temperature and pressure control system. The temperature control accuracy is ±0.5℃, and the pressure control accuracy is ±0.05MPa. The reactor's temperature control system is activated, and the temperature inside the reactor is raised to 31℃ at a rate of 1℃ / min using an electric heating device. During the heating process, the stirring device is kept running at a low speed. After the temperature stabilizes, food-grade liquid CO2 is added to the reactor using a high-pressure pump, increasing the pressure inside the reactor to 7.39MPa at a rate of 0.5MPa / min. At this point, the CO2 reaches a supercritical state.

[0029] After the temperature and pressure inside the reactor stabilized, the pressurization and heating devices were turned off, and the supercritical state was maintained for 6 hours. During the heat preservation process, the stirring device was turned on every 2 hours, with a stirring rate of 150 r / min, and stirring was continued for 15 minutes before being turned off to allow the materials to react fully. During the heat preservation process, supercritical CO2, with its high diffusivity, penetrated into the three-dimensional network pores of the silica wet gel, fully replacing and dissolving the water inside the wet gel. At the same time, the low-temperature lipase played a catalytic role, promoting a stable bonding reaction between the functional groups of the enzymatically hydrolyzed lignin derivatives and the silanol groups on the silica surface, completing the enzymatically catalyzed hydrophobic modification. The supercritical CO2 dried the gel under the condition of no surface tension, effectively preserving the three-dimensional network structure of silica, and simultaneously completing the integrated treatment of wet gel water replacement, enzymatically catalyzed hydrophobic modification, and supercritical fluid drying.

[0030] Pressure relief and CO2 recovery: After the supercritical integrated treatment is completed, the multi-stage throttling pressure relief device of the reactor is turned on for pressure relief. The device is a series throttling valve structure. The pressure relief rate is strictly controlled at 0.1MPa / min by adjusting the valve opening. During the pressure relief process, the temperature inside the reactor is kept stable at 30℃ by the temperature control system until the pressure inside the reactor drops to atmospheric pressure.

[0031] During the depressurization process, the gaseous CO2 discharged from the reactor is introduced into the condensation recovery system through pipelines. First, the gaseous CO2 is passed into the condenser and condensed into liquid CO2 under process conditions of -10℃ and 2MPa. Then, the liquid CO2 is passed into an activated carbon adsorption tower and a molecular sieve purification tower in sequence. The activated carbon adsorption tower removes trace organic impurities from the liquid CO2, and the molecular sieve purification tower removes moisture and inorganic impurities from the liquid CO2. The purity of the purified CO2 is tested to be no less than 99.9%, and it is stored in a high-pressure storage tank for recycling. In this embodiment, the CO2 recovery rate is calculated to be 95%.

[0032] Finished Product Inspection: After the reactor was completely depressurized and returned to normal temperature and pressure, the reactor door was opened, and the product inside was carefully removed. The product was a white, lightweight aerogel material with no obvious structural damage. Multiple performance tests were performed on the product. The water contact angle test used the seated drop method with deionized water at 25℃. Five test points were selected at different locations on the product, and the average value was taken. The initial water contact angle of the product was 140°. The compressive strength test used a universal testing machine. The product was processed into a standard 20mm×20mm×20mm cube sample, and the test rate was set to 1mm / min. The compressive strength of the product was 1.7MPa. The ultra-low temperature stability test placed the product in an ultra-low temperature chamber at -120℃ with a temperature fluctuation range of ±2℃. The sample was left without external load for 1000 hours. The water contact angle of the product was 135°, the compressive strength retention rate was 90%, and the sample showed no obvious cracking or deformation. All performance tests were passed, resulting in a biomass-reinforced hydrophobic silica aerogel product suitable for low-temperature environments. Example

[0033] Wheat straw and corn stalks were mixed at a 1:1 mass ratio as agricultural waste raw materials. After removing impurities and cutting the mixture into sections, it was placed in a muffle furnace and burned in air at a controlled temperature of 600℃ for 3 hours. After natural cooling to room temperature, straw ash was obtained. The straw ash was then pulverized using a universal pulverizer and passed through a 100-mesh standard inspection sieve. The material passing through the sieve was considered qualified straw ash, and its silica mass fraction was determined to be 88% by gravimetric analysis.

[0034] Qualified straw ash and a 15% sodium hydroxide solution were mixed at a solid-liquid ratio of 1:7. The mixture was then transferred to a reaction vessel with a constant-temperature water bath stirrer. The water bath temperature was set to 88℃, and the stirring speed was 400 r / min. The mixture was stirred continuously for 4 hours. After the reaction, the mixture was filtered through a 150-mesh filter to remove insoluble impurities, yielding a sodium silicate solution. A 10% dilute sulfuric acid solution was added dropwise to the sodium silicate solution, and the mixture was stirred continuously until the pH value of the solution reached 4 and hydrated silicic acid precipitated. The solution was then allowed to stand and age in a 25℃ constant-temperature incubator for 18 hours.

[0035] After aging, the material was filtered and repeatedly washed with deionized water. The conductivity of the filtrate was measured using a conductivity meter after each wash until the conductivity was below 100 μS / cm. The filtrate was then dispersed in deionized water to obtain a highly active silica sol, with a silica mass fraction of 15% determined by gravimetric analysis. Ammonia solution with a mass fraction of 1.2% was added dropwise to the silica sol to adjust the pH to 9. The mixture was then allowed to gel at a constant temperature of 30°C for 2 hours to form a transparent gel. After gelation, the gel was transferred to an electrically heated constant temperature oven and aged at 50°C for 36 hours to obtain a silica wet gel. The solid content was measured to be 10%, and the porosity was 93%. The prepared silica wet gel was then completely transferred into the reaction chamber of a supercritical reactor. During the loading process, the three-dimensional network structure of the wet gel was kept intact, and no solvent exchange was performed. Figure 2 As shown.

[0036] Construction of solvent-free system: Alkali lignin was selected as the raw material for lignin derivatives. It was placed in a Soxhlet extractor and 95% ethanol solution was added. The mixture was refluxed and washed 4 times under a water bath at 55°C for 1.5 hours each time. After washing, the material was taken out and dried in an electric constant temperature oven at 108°C for 3 hours. After pulverization, it was passed through a 90-mesh sieve to obtain activated alkali lignin derivatives.

[0037] Food-grade liquid CO2 was introduced into a supercritical reactor, and the initial temperature and pressure of the reactor were controlled at 15℃ and 4MPa, respectively. The purity of the liquid CO2 was not less than 99.9%. The activated alkali lignin derivative and low-temperature lipase were added. The amount of activated alkali lignin derivative added was 7.5% of the mass of the introduced CO2, and the activity of the low-temperature lipase was 12000 U / g. The optimal reaction temperature was 0℃~35℃, and the amount added was 1.2% of the mass of the silica wet gel. A mechanical stirrer was turned on at a stirring speed of 300 r / min and stirred continuously for 45 min to ensure that all materials were uniformly dispersed in the liquid CO2, thus constructing a homogeneous reaction system without organic solvents or synthetic modifiers.

[0038] Supercritical integrated treatment: A stainless steel supercritical reactor is used, with temperature control accuracy ±0.5℃ and pressure control accuracy ±0.05MPa. The temperature control system is turned on, and the temperature is increased to 31℃ at a rate of 1.5℃ / min. Then, liquid CO2 is added, and the pressure is increased to 7.39MPa at a rate of 0.8MPa / min, so that the CO2 reaches the supercritical state. After the temperature and pressure stabilize, the supercritical state is maintained for 7 hours. During the holding period, the stirring device is turned on once every 1.5 hours at a stirring rate of 220r / min for 12 minutes to ensure that the materials react completely.

[0039] Under supercritical conditions, supercritical CO2 permeates into the pores of the wet gel to replace moisture. Low-temperature lipase catalyzes the bonding reaction between alkali lignin derivatives and silanol groups on the surface of silica to achieve hydrophobic modification. At the same time, supercritical CO2 completes gel drying under no surface tension conditions, preserving its three-dimensional network structure and simultaneously completing the integrated processing of the three core processes.

[0040] Pressure Relief and CO2 Recovery: After the supercritical integrated treatment is completed, a multi-stage throttling pressure relief device is activated, controlling the pressure relief rate at 0.2 MPa / min. During the pressure relief process, the temperature inside the reactor is maintained at 31°C until the pressure inside the reactor drops to atmospheric pressure. The discharged gaseous CO2 is introduced into a condensation and recovery system, where it is condensed into liquid CO2 at 0°C and 2.5 MPa. After impurities are removed by an activated carbon adsorption tower and a molecular sieve purification tower, the purified CO2 purity is not less than 99.9%, and it is stored in a high-pressure storage tank for recycling. In this embodiment, the CO2 recovery rate is 97%.

[0041] Finished product testing: After the reactor returned to normal temperature and pressure, the product was removed. The product was a white, lightweight aerogel with no structural damage. Performance testing was performed on the product. The average water contact angle using the drop test was 142°, and the compressive strength was 1.8 MPa. After placing the product in a -120°C ultra-low temperature environment chamber for 1000 hours, the water contact angle was measured at 137°, and the compressive strength retention rate was 92%. The sample showed no cracks or deformation, and all performance indicators met the standards, resulting in a biomass-reinforced hydrophobic silica aerogel product. Example

[0042] Wet gel preparation: Rice straw, wheat straw, and corn straw were mixed in a mass ratio of 1:1:1 as agricultural waste raw materials. After impurities were removed and the mixture was cut into sections, it was placed in a muffle furnace and incinerated in an air atmosphere. The incineration temperature was controlled at 700℃, and the incineration time was 2 hours. After natural cooling, straw ash was obtained. The straw ash was crushed and passed through a 120-mesh standard inspection sieve. The material passing through the sieve was qualified straw ash, and its silica mass fraction was determined to be 90% by gravimetric analysis.

[0043] Qualified straw ash and a 20% sodium hydroxide solution were mixed at a solid-liquid ratio of 1:10. The mixture was then transferred to a reaction vessel in a constant-temperature water bath with a stirrer. The water bath temperature was set to 95℃, and the stirring speed was 500 r / min. The mixture was stirred continuously for 3 hours. After the reaction, the mixture was filtered through a 200-mesh filter to obtain a sodium silicate solution. A 15% dilute sulfuric acid solution was added dropwise to the sodium silicate solution, and the mixture was stirred until the pH value reached 5 and hydrated silicic acid precipitated. The mixture was then allowed to stand and age in a 30℃ constant-temperature incubator for 12 hours.

[0044] After aging, the material was filtered and washed with deionized water. The conductivity of the filtrate was measured using a conductivity meter after each wash until the conductivity was below 100 μS / cm. The filtrate was then dispersed in deionized water to obtain a highly active silica sol, with a silica mass fraction of 20% determined by gravimetric analysis. Ammonia solution (2% by mass) was added dropwise to the silica sol to adjust the pH to 10. The mixture was then allowed to gel at a constant temperature of 35°C for 1 hour to form a transparent gel. After gelation, the gel was transferred to an electric thermostatic oven and aged at 60°C for 24 hours to obtain a wet silica gel. The wet silica gel had a solid content of 15% and a porosity of 95%. The prepared wet silica gel was then completely transferred into the reaction chamber of a supercritical reactor, ensuring the three-dimensional network structure of the wet gel remained intact during loading, without any solvent exchange.

[0045] Construction of solvent-free system: Sulfonated lignin was selected as the raw material for lignin derivatives. It was placed in a Soxhlet extractor and 95% ethanol solution was added. The mixture was refluxed in a water bath at 60°C for 3 times, each time for 2 hours. After washing, it was dried in an electric thermostatic oven at 110°C for 2 hours. After pulverization, it was passed through a 100-mesh sieve to obtain activated sulfonated lignin derivatives.

[0046] Food-grade liquid CO2 was introduced into a supercritical reactor, with the initial temperature controlled at 20℃ and the initial pressure at 5MPa. The purity of the liquid CO2 was not less than 99.9%. Activated sulfonated lignin derivatives and low-temperature lipase were added. The amount of activated sulfonated lignin derivatives added was 10% of the mass of the introduced CO2. The activity of the low-temperature lipase was 15000U / g, the optimal reaction temperature was 0℃~35℃, and the amount added was 2% of the mass of the silica wet gel. A mechanical stirrer was turned on at a stirring speed of 400r / min and stirred continuously for 30min to ensure uniform dispersion of the materials, thus constructing a homogeneous reaction system without organic solvents or synthetic modifiers.

[0047] Supercritical integrated treatment: A stainless steel supercritical reactor was used, with temperature control accuracy ±0.5℃ and pressure control accuracy ±0.05MPa. The temperature control system was activated, and the temperature was increased to 31℃ at a rate of 2℃ / min. Liquid CO2 was added, and the pressure was increased to 7.39MPa at a rate of 1MPa / min, bringing the CO2 to a supercritical state. After the temperature and pressure stabilized, the supercritical state was maintained for 8 hours. During the maintenance process, the stirring device was activated once every hour at a stirring rate of 300 rpm for 10 minutes to ensure complete reaction.

[0048] Under supercritical conditions, supercritical CO2 completes the water replacement of the wet gel, low-temperature lipase catalyzes the bonding of sulfonated lignin derivatives with silanol groups on the silica surface to achieve hydrophobic modification, and supercritical CO2 drying without surface tension preserves the three-dimensional network structure of the gel, thus completing the integrated treatment simultaneously.

[0049] Pressure Relief and CO2 Recovery: After the supercritical integrated treatment is completed, a multi-stage throttling pressure relief device is activated, controlling the pressure relief rate at 0.3 MPa / min. During the pressure relief process, the temperature inside the reactor is maintained at 32°C until the pressure drops to atmospheric pressure. The discharged gaseous CO2 condenses into liquid at -5°C and 3 MPa. After purification by an activated carbon adsorption tower and a molecular sieve purification tower, the CO2 purity is not less than 99.9%, and it is stored in a high-pressure storage tank for recycling. In this embodiment, the CO2 recovery rate is 98%.

[0050] Finished product testing: After the reactor returned to normal temperature and pressure, the product was removed. It was a white, lightweight aerogel with no structural damage. Performance testing results showed that the initial water contact angle of the product was 145°, and the compressive strength was 1.9 MPa. After being placed in an ultra-low temperature environment of -120℃ for 1000 hours, the water contact angle was 139°, and the compressive strength retention rate was 95%. The sample showed no cracking or deformation, and all performance indicators met the standards, resulting in a biomass-reinforced hydrophobic silica aerogel product.

[0051] Comparative example: Preparation of wet gel and solvent exchange: Rice straw was selected as raw material and silica wet gel was prepared according to the process in Example 1. The wet gel was transferred into anhydrous ethanol for solvent exchange. The anhydrous ethanol was replaced every 8 hours for a total of 4 times. After the solvent exchange was completed, the wet gel was transferred into a supercritical reactor.

[0052] Construction of organic solvent system: Anhydrous ethanol was added to a supercritical reactor as the reaction medium, and silane chemical modifiers were added as hydrophobic modifiers. No lignin derivatives or low-temperature enzyme catalysts were added. Stirring was started to disperse the materials, thus constructing a reaction system with organic solvent as the medium.

[0053] Stepwise modification and drying: The temperature of the reactor was adjusted to 60℃ and the pressure was adjusted to 5MPa. The reactor was kept at this temperature for 4 hours to complete the chemical hydrophobic modification. After the modification was completed, the process parameters of the reactor were adjusted to the supercritical state of CO2 and the reactor was kept at this temperature for 6 hours for supercritical drying. The hydrophobic modification and supercritical drying were carried out in steps.

[0054] Pressure relief and material discharge: After modification and drying, the pressure relief valve of the reactor was opened directly to relieve pressure without controlling the pressure relief rate. During the pressure relief process, CO2 was not recovered and the gaseous CO2 mixed with organic solvent was directly discharged to the treatment device.

[0055] Product testing: After removing the product from the reactor, performance testing was conducted. The initial water contact angle of the product was 125° and the compressive strength was 1.2 MPa. After placing the product in a -120°C ultra-low temperature environment chamber for 1000 hours, the water contact angle dropped to 110°, the compressive strength retention rate was only 65%, obvious cracks appeared on the sample surface, and structural deformation occurred in some areas. All properties failed to meet the requirements for use in low-temperature environments.

[0056] Comparison of process parameters and product performance: To clearly demonstrate the selection of process parameters and the core performance of the products in each embodiment of the present invention, and to intuitively compare the differences between the process of the present invention and the traditional process, the key process parameters and product performance test results of the above three embodiments and one comparative example are summarized in the following simplified table. All data are obtained from actual testing and calculation, and objectively reflect the correlation between the process and the product.

[0057] project Example 1 Example 2 Example 3 Comparative Example Agricultural waste raw materials rice straw wheat straw + corn stalks Rice straw + wheat straw + corn straw rice straw Lignin derivative types Enzymatic hydrolysis of lignin Alkali lignin Sulfonated lignin No additions Low-temperature enzyme catalyst addition amount 0.5% of wet gel mass The wet gel mass is 1.2%. 2% of wet gel mass No additions Supercritical integrated treatment heat preservation time 6h 7h 8h Modification for 4 hours + drying for 6 hours pressure relief rate 0.1 MPa / min 0.2MPa / min 0.3MPa / min Direct pressure relief without speed control <![CDATA[CO2 recovery rate]]> 95% 97% 98% 0% (Not recycled) Initial water contact angle 140° 142° 145° 125° compressive strength 1.7MPa 1.8MPa 1.9MPa 1.2MPa Water contact angle after ultra-low temperature 135° 137° 139° 110° Compressive strength retention rate after ultra-low temperature 90% 92% 95% 65% Appearance after ultra-low temperature No cracks or deformation No cracks or deformation No cracks or deformation Surface cracking, structural deformation The table above clearly presents the correspondence between the changes in process parameters and product performance in different embodiments of the present invention, and also clearly demonstrates the difference between the traditional process and the process of the present invention. All three embodiments of the present invention strictly follow the process features of the claims. From the resource extraction of silicon sources from agricultural waste, to the construction of a supercritical CO2 solvent-free homogeneous reaction system, and then to the realization of integrated supercritical treatment and efficient CO2 recovery, each step forms a complete technical system. The products prepared all meet the performance requirements of hydrophobic silica aerogels in low-temperature environments, and with the optimization of process parameters within a reasonable range, the product performance shows a steady upward trend.

[0058] In contrast, the comparative example, lacking the core process features of this invention, employs a traditional stepwise process involving solvent exchange, organic solvent media, and chemical modifiers. This not only fails to achieve resource recovery during the preparation process, increasing environmental impact and production costs, but also results in products with significant deficiencies in hydrophobic properties, mechanical strength, and ultra-low temperature stability, failing to meet the practical application requirements of low-temperature environments. This comparative result fully demonstrates the rationality and practicality of the process of this invention. The synergistic effect of each process feature can effectively improve the overall performance of the product, while simultaneously achieving a green and resource-efficient preparation process, possessing a solid foundation for industrial application.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing hydrophobic silica aerogel for low-temperature environments, characterized in that, The specific steps of this method are as follows: S100, Wet Gel Preparation and Loading: Active silicon source is extracted from agricultural waste as raw material, and highly active silica sol is obtained through alkali dissolution, acidification, aging and washing. Silica wet gel is prepared by sol-gel method. The prepared wet gel is directly loaded into supercritical reactor. No solvent exchange operation between organic solvent and water is performed throughout the process. S200, Solvent-free system construction: Liquid CO2 is introduced into a supercritical reactor containing silica wet gel, and purified and activated lignin derivatives and low-temperature enzyme catalysts are added at the same time. Stirring is turned on to make the materials uniformly dispersed in CO2, thus constructing a solvent-free homogeneous reaction system with supercritical CO2 as the medium. S300, supercritical integrated treatment: The process parameters of the supercritical reactor are adjusted to the temperature and pressure at which CO2 reaches the supercritical state, and the supercritical state of CO2 is maintained for 6-8 hours. Simultaneously, the wet gel water replacement, enzyme-catalyzed hydrophobic modification and supercritical fluid drying are completed. S400, depressurization and CO2 recovery: After the supercritical integrated treatment is completed, the pressure inside the reactor is slowly reduced to atmospheric pressure, and the purified CO2 is recovered simultaneously during the depressurization process; S500 Finished Product Discharge Inspection: After the reactor is completely depressurized to room temperature and pressure, the product inside the reactor is taken out and its performance is tested. After the performance test, a biomass-reinforced hydrophobic silica aerogel product suitable for low temperature environment is obtained.

2. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S100, the agricultural waste is one of rice straw, wheat straw, and corn straw. The agricultural waste is incinerated in an air atmosphere to obtain straw ash. The incineration process parameters are incineration temperature of 500℃~700℃ and incineration time of 2 hours to 4 hours. The straw ash obtained after incineration is crushed and passed through an 80-120 mesh sieve. The mass fraction of silicon dioxide in the straw ash after sieving is not less than 85%.

3. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S100, the extraction of active silicon source requires mixing straw ash with a 10%–20% sodium hydroxide solution at a solid-liquid ratio of 1:5–1:

10. The mixture is stirred at a rate of 300–500 r / min for 3–5 hours in a water bath at 80–95°C. After the reaction, the mixture is filtered through a 100–200 mesh screen to remove insoluble carbon residue and impurities, yielding a sodium silicate solution. A 5%–15% dilute sulfuric acid solution is added dropwise to the sodium silicate solution to adjust the pH to 3–5, causing silicon dioxide to precipitate as hydrated silicic acid. After precipitation, the solution is allowed to stand and age at 20–30°C for 12–24 hours. After aging, the solution is repeatedly washed with deionized water until the conductivity of the filtrate is below 100 μS / cm, yielding a highly active silica sol with a silicon dioxide mass fraction of 10%–20%.

4. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S100, the process of preparing silica wet gel by sol-gel method involves adding 0.5% to 2% ammonia water by mass fraction to a highly active silica sol as a gel catalyst, adjusting the pH value of the silica sol to 8 to 10, and allowing it to stand and gel at a constant temperature of 25℃ to 35℃ for 1 to 3 hours. After the silica sol has completely formed a transparent gel, it is aged in a constant temperature oven at 40℃ to 60℃ for 24 to 48 hours to obtain silica wet gel with a continuous three-dimensional network structure. The solid content of the wet gel is 5% to 15%, and the porosity of the wet gel is not less than 90%.

5. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S200, the lignin derivative is one of enzymatically hydrolyzed lignin, alkali lignin, and sulfonated lignin. The lignin derivative is purified and activated by reflux washing the lignin raw material with a 95% ethanol solution at 50℃~60℃ 3 to 5 times, with each washing time lasting 1h~2h, to remove residual sugars, hemicellulose, and ash impurities from the raw material. After washing, the raw material is dried in an oven at 105℃~110℃ for 2h~4h, pulverized, and passed through an 80-100 mesh sieve to obtain the activated lignin derivative. The amount of lignin derivative added is 5%~10% of the mass of CO2 introduced.

6. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S200, the low-temperature enzyme catalyst is a low-temperature lipase, the optimal reaction temperature of which is 0℃~35℃, the enzyme activity is not less than 10000U / g, and the addition amount is 0.5%~2% of the mass of the silica wet gel. The introduced CO2 is food-grade liquid CO2 with a purity of not less than 99.9%. The initial temperature in the supercritical reactor when introducing CO2 is 10℃~20℃ and the initial pressure is 3MPa~5MPa. During the introduction process, the stirring device of the reactor is turned on, the stirring rate is 200r / min~400r / min, and the stirring time is 30min~60min, forming a homogeneous reaction system that is completely free of organic solvents and synthetic modifiers.

7. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S300, the supercritical reactor is made of stainless steel with a temperature control accuracy of ±0.5℃ and a pressure control accuracy of ±0.05MPa. When adjusting the process parameters, the temperature is first increased to 31℃ at a rate of 1℃ / min to 2℃ / min, and then the pressure is increased to 7.39MPa at a rate of 0.5MPa / min to 1MPa / min. After the temperature and pressure inside the reactor stabilize, the supercritical state is maintained for 6h to 8h. During the heat preservation process, the stirring device is turned on every 1h to 2h and stirred at a rate of 150r / min to 300r / min for 10min to 15min.

8. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S300, the wet gel water replacement involves permeating supercritical CO2 into the three-dimensional network pores of the silica wet gel to replace the water inside the wet gel; the enzyme-catalyzed hydrophobic modification involves a bonding reaction between lignin derivatives and silanol groups on the silica surface under the action of low-temperature enzymes; and the supercritical fluid drying involves drying the gel with supercritical CO2 under conditions of no surface tension, while preserving the three-dimensional network structure of silica.

9. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S400, during the depressurization process, the temperature inside the reactor is kept stable at 30℃~32℃, and the depressurization rate is strictly controlled at 0.1MPa / min~0.3MPa / min until the pressure inside the reactor drops to atmospheric pressure. CO2 recovery involves condensing the gaseous CO2 discharged during depressurization into liquid state in a condenser at -10℃ to 0℃ and 2MPa~3MPa. Then, impurities are removed by an activated carbon adsorption tower and a molecular sieve purification tower. The purity of CO2 after purification is not less than 99.9%, and the CO2 recovery rate is not less than 95%.

10. The method for preparing a hydrophobic silica aerogel for low-temperature environments according to claim 1, characterized in that, In step S500, the performance testing includes water contact angle testing, compressive strength testing, and cryogenic stability testing. The water contact angle test uses the seated drop method, with deionized water as the test solution and a test temperature of 25°C. The initial water contact angle of the aerogel is not less than 140°. The compressive strength test uses a universal testing machine with a sample size of 20mm×20mm×20mm and a test rate of 1mm / min. The compressive strength of the aerogel is not less than 1.7MPa. The cryogenic stability test involves placing the aerogel in a cryogenic chamber at -120°C for 1000 hours. The temperature fluctuation range of the chamber is ±2°C. During the test, the sample is not subjected to any external force load. After the test, the water contact angle of the aerogel is not less than 135°, the compressive strength retention rate is not less than 90%, and the sample shows no obvious cracking or deformation.

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