Environment-friendly preparation method and system of high-porosity cellulose porous membrane based on solvent replacement

Abstract

The invention belongs to the technical field of biomass materials and green chemical engineering, and discloses a green preparation method and system of a high-porosity cellulose porous membrane based on solvent replacement. The invention aims to solve the problems that the pore structure is easy to dry and collapse, the solvent cost is high and the environment is not protected when the cellulose porous membrane is prepared in the prior art. The core of the method is that cellulose is dissolved in a lithium bromide aqueous solution, and a cellulose hydrogel network is formed through cooling solidification and washing regeneration; then, completely replacing the hydrogel by using a low-surface-tension alcohol or ketone solvent (such as tertiary butanol) so as to eliminate destructive capillary force in the drying process and maintain a porous structure; and finally, drying at normal pressure to obtain the high-porosity cellulose membrane. The core of the system is that a three-loop recycling system of lithium bromide and alcohol or ketone solvents is integrally designed, and near-zero emission and cyclic utilization of the solvents are achieved. The prepared cellulose porous membrane is high in porosity (50-80%) and controllable in structure, and the whole process is green, economical and suitable for large-scale production.

Description

A green preparation method and system for high-porosity cellulose porous membranes based on solvent displacement Technical Field

[0001] This invention belongs to the field of biomass materials and green chemical technology, specifically relating to a green preparation method of cellulose porous membrane with high porosity and controllable structure based on solvent replacement, the product obtained thereby, and a solvent closed-loop circulation system for realizing this method. Background Technology

[0002] Cellulose porous membranes hold significant promise for applications in separation and purification, energy devices (such as battery separators), and bioengineering. However, traditional preparation methods, such as dissolution-regeneration, often face two major technical bottlenecks: First, during the conventional drying process of wet gels, the high surface tension of water generates enormous capillary forces, leading to severe collapse of the delicate micro / nanopore structure, typically resulting in only dense membranes or membranes with extremely low porosity. To maintain the pore structure, expensive and energy-intensive specialized technologies such as freeze-drying or supercritical drying must be employed, severely restricting their large-scale application. Second, commonly used cellulose solvents (such as N-methylmorpholine-N-oxide, ionic liquids, and strong base / urea systems) are expensive, difficult to recycle, toxic, or environmentally unfriendly, resulting in high production costs and significant environmental pressure.

[0003] In existing technologies, there are also template methods for preparing large-pore films using the addition of porogens (such as soluble salt particles) (e.g., patent document CN201910636225.9). While this method can obtain larger pores, its pore-forming mechanism depends on the physical occupancy of solid particles and subsequent dissolution. The shape, size, and distribution of the prepared pores are severely limited by the porogen particles themselves, making it difficult to obtain a three-dimensional, interconnected nanofiber network structure formed by phase separation. More importantly, such methods completely fail to address the common problem of drying collapse, nor do they design efficient recovery and recycling of the large amount of chemical solvent used for dissolution. Essentially, it remains a "linear" consumable process.

[0004] Therefore, developing an integrated technology and system that can effectively maintain the microporous structure of cellulose membranes under mild conditions and achieve green solvent recycling throughout the entire preparation process has outstanding technological innovation value and practical economic and environmental significance. Summary of the Invention

[0005] The present invention aims to overcome the aforementioned deficiencies of the prior art. Its primary objective is to provide a method for preparing high-porosity cellulose porous membranes under normal pressure drying conditions through a precisely controlled solvent replacement step. Another parallel objective of the present invention is to provide an integrated closed-loop solvent recovery and recycling system, making the preparation method more economically and environmentally sustainable.

[0006] To achieve the above objectives, the present invention provides a green preparation method for a high-porosity cellulose porous membrane based on solvent replacement, the product obtained by the method, and a dedicated solvent closed-loop circulation system.

[0007] In a first aspect, the present invention provides a green preparation method for a high-porosity cellulose porous membrane based on solvent displacement, comprising the following steps: (1) Dissolution: dispersing cellulose raw materials (such as microcrystalline cellulose, pulp) in a lithium bromide aqueous solution of a specific concentration, and dissolving them under heating and stirring to obtain a uniform and transparent cellulose solution. Lithium bromide is a highly efficient, mild, and water-washable regenerable cellulose solvent. It is the basis of the green cycle of this scheme.

[0008] (2) Molding: The cellulose solution is molded by casting, and then cooled to below 70°C to solidify, resulting in a solidified cellulose gel. No cross-linking agents or other substances need to be added. This process solidifies the cellulose solution.

[0009] (3) Regeneration washing: The solidified cellulose gel is regenerated by countercurrent rinsing (industrial preparation process) or immersion in flowing deionized water (laboratory preparation process) to induce phase separation, thereby regenerating cellulose and simultaneously removing lithium bromide to obtain cellulose hydrogel. This process involves water to phase separation, with cellulose precipitating from the solution to form a cellulose-rich three-dimensional gel network. This network structure lays the foundation for the final porous membrane. The gel is then washed with water to completely remove lithium bromide.

[0010] (4) Solvent replacement: The washed gel is sequentially immersed in alcohol or ketone solvents with a surface tension significantly lower than that of water to achieve a thorough replacement. This step is one of the core inventive aspects of this invention. Its function is to replace the water (~72 mN / m) with high surface tension in the gel channels with low surface tension alcohol or ketone solvents (such as tert-butanol ~20.7 mN / m), thereby reducing the capillary force in the subsequent drying process by several times and fundamentally preventing the collapse of the channels.

[0011] (5) Drying: Dry the replaced gel under normal or reduced pressure to obtain a cellulose porous membrane with intact structure.

[0012] Preferably, the green preparation method further includes: (6) solvent recovery and recycling: collecting the lithium bromide-containing wastewater from the washing step (3), concentrating and purifying it by evaporation or membrane separation, and then recycling it back to the dissolution step (1); collecting the aqueous alcohol or ketone solvent from the displacement step (4), dehydrating and purifying it by molecular sieve adsorption, and then recycling it back to the displacement step (4); collecting the volatile alcohol or ketone solvent from the drying step (5), and recycling it back to the displacement step (4) through a recycling pipeline. This three-loop recycling system constitutes another creative core of the present invention, realizing the closed-loop and greening of the process.

[0013] Preferably, in step (1), the mass concentration of the lithium bromide aqueous solution is 55%-65%; and the dissolution temperature is 110℃-140℃.

[0014] Preferably, in step (1), the cellulose raw material is microcrystalline cellulose or pulp; and the mass ratio of the cellulose raw material to the lithium bromide aqueous solution is 1:18~20.

[0015] Preferably, in step (2), the cooling and solidification time is 20~60 min; the cooling and solidification temperature is 10~35℃.

[0016] Preferred: In step (3), the solidified cellulose gel is treated by countercurrent rinsing or immersion in flowing deionized water for 24 hours to completely remove lithium bromide.

[0017] Preferably, in step (4), the alcohol or ketone solvent is at least one of methanol, ethanol, isopropanol, tert-butanol or acetone; more preferably, the solvent is tert-butanol.

[0018] Preferred: In step (4), the cellulose hydrogel is subjected to solvent replacement by displacement, and finally replaced with a cellulose hydrogel containing only alcohol or ketone solvents.

[0019] Preferably, in step (4), the degree of solvent replacement is based on the residual moisture content inside the gel after replacement being less than 1%.

[0020] Preferably, in step (6), when collecting the lithium bromide-containing wastewater from the washing step (3), the concentration is carried out using evaporation or membrane separation technology; when collecting the water-containing alcohol or ketone solvent from the displacement step (4), the dehydration and purification is carried out using molecular sieve adsorption technology.

[0021] In a second aspect, the present invention provides a high-porosity cellulose porous membrane prepared by the method described in the first aspect, wherein the porosity of the porous membrane is 50-80%.

[0022] Thirdly, the present invention provides an application of the high-porosity cellulose porous membrane prepared by the method described in the first aspect in the preparation of battery separators and ion filtration membranes.

[0023] Fourthly, the present invention provides a solvent closed-loop circulation system for implementing the method described in the first aspect, characterized in that it comprises: a lithium bromide solution recovery and purification unit, an alcohol solvent recovery and purification unit, a fluid transport pipeline, and a process control unit; the lithium bromide solution recovery and purification unit is used to treat washing wastewater and return it to the dissolution process, and the alcohol solvent recovery and purification unit is used to treat aqueous alcohol solution and return it to the displacement process.

[0024] Compared with existing technologies, the beneficial effects of this invention are: 1. Excellent structure retention: Through a unique low surface tension solvent replacement technology, the problem of collapse of cellulose porous membranes during atmospheric pressure drying has been successfully solved. Without expensive equipment, the membrane porosity can be increased from 10% in direct drying to 60-80%, and a continuous three-dimensional nanofiber network can be formed.

[0025] 2. A complete green manufacturing system: This invention is the first to deeply integrate the lithium bromide dissolution system with targeted solvent recovery technology. Lithium bromide can achieve a high efficiency of >95% recovery and multiple recycling with alcohols or ketones, reducing chemical waste emissions at the source, significantly lowering raw material costs, and embodying the concept of a circular economy.

[0026] 3. Excellent and controllable product performance: By adjusting parameters such as cellulose concentration, lithium bromide concentration, dissolution temperature, and type of replacement solvent, the pore size distribution and mechanical strength of the membrane can be finely adjusted within a certain range to meet the needs of different application scenarios, such as battery separators and ion filtration membranes.

[0027] 4. Significant industrialization advantages: The main operating units of the entire process chain (dissolution, displacement, drying, and recovery) are all common chemical equipment, which are easy to integrate and scale up production, providing a practical and feasible technical solution for the large-scale manufacturing of high-performance cellulose porous membranes. Attached Figure Description

[0028] Figure 1 shows the recovered solvent and its Fourier transform infrared spectrum during the implementation of this invention; where a: digital image of the recovered lithium bromide solution; b: Fourier transform infrared spectra of fresh and recovered lithium bromide solutions; c: the recovered lithium bromide solution can re-dissolve cellulose; d: left is tert-butanol (containing water) after displacement, right is tert-butanol after dehydration by molecular sieve; e: Fourier transform infrared spectra of commercially available tert-butanol and recovered tert-butanol.

[0029] Figure 2 is a SEM image of the cellulose porous membrane obtained in Example 1 of the present invention (drying after replacement with tert-butanol) (a. cross section, b. surface).

[0030] Figure 3 shows the test results of thermal stability and biodegradability of CE film and PP film in Example 3 of the present invention; a: Pictures of PP film before and after 1 hour of heat treatment; b: Pictures of CE film before and after 1 hour of heat treatment; c: Pictures of PP film before and after 1 month after being buried in soil; d: Pictures of CE film before and after 1 month after being buried in soil.

[0031] Figure 4 shows scanning electron microscope (SEM) images of the cellulose membrane obtained in Comparative Example 1 (no solvent replacement, direct drying) (a. cross section, b. surface).

[0032] Figure 5 shows a scanning electron microscope (SEM) image of the membrane obtained in Comparative Example 2.

[0033] Figure 6 is a complete process flow diagram of the preparation process of the present invention and a schematic diagram of the factory process of the solvent circulation system. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0035] Example 1: Standard Procedure and Solvent Cycle Validation 1. Dissolution: Weigh 10g of microcrystalline cellulose, add 190g of 60wt% lithium bromide aqueous solution, stir and dissolve at 120℃ for 20 minutes to obtain a homogeneous solution.

[0036] 2. Shaping and solidification: After degassing, the solution is scraped onto a glass plate and placed in an environment of 25°C to solidify for 30 minutes to form a gel film.

[0037] 3. Washing: Immerse the gel membrane in flowing deionized water for 24 hours to completely remove lithium bromide.

[0038] 4. Solvent replacement: The hydrogel was sequentially placed in 100% tert-butanol for replacement, each time for 12 hours, until a complete and thorough replacement was achieved. The degree of solvent replacement was determined by the residual water content inside the gel being less than 1% by mass after replacement.

[0039] 5. Drying: Place in a 60℃ oven and dry under normal pressure for 12 hours to obtain a flexible white porous membrane (labeled as membrane A).

[0040] 6. Solvent Recovery: The washing wastewater is combined and concentrated by heating to a lithium bromide concentration of approximately 60%. The recovered lithium bromide solution can be reused to dissolve cellulose. The mixed aqueous tert-butanol is dehydrated and recovered using a molecular sieve. The recovered tert-butanol can be reused for solvent replacement (corresponding to Figure 1). The recovered solvent is used for the next batch of membrane production, and the cycle is repeated 3 times.

[0041] Test results: The obtained membrane is semi-transparent. SEM images (corresponding to Figure 2) show that its structure is loose and porous, exhibiting a highly interconnected three-dimensional nanofiber network porous structure with a porosity of 80% and an average pore size of 1.2 μm.

[0042] Strain, stress, and tensile modulus tests: A universal testing machine was used. The samples were cut into strips 10 mm wide, with a gauge length of 50 mm and a tensile speed of 5 mm / min. Five parallel samples were tested for each set of data, and the average value was taken. The strain, stress, and tensile modulus of the membrane were 6.6%, 12.2 MPa, and 398.3 MPa, respectively. The membrane performance obtained after the third cycle showed a fluctuation of less than 5% compared to the first cycle (corresponding to Table 1).

[0043] Table 1 Mechanical properties of cellulose membranes prepared with fresh and recycled reagents

[0044] Example 2: The operation was the same as in Example 1, except that the replacement solvent in step 4 was changed to: a) only 100% ethanol was used for replacement.

[0045] b) Replacement is performed using only 100% acetone.

[0046] Test results: a) The obtained membrane porosity was 50%, but there was slight shrinkage; b) The obtained membrane porosity was 64%. This proves that tert-butanol has advantages in overall performance.

[0047] Example 3: Battery Separator Application Following steps 1-5 of Example 1, a porous cellulose membrane (denoted as CE membrane) was prepared. The dried CE membrane was punched and cut for later use. A commercially available polypropylene membrane (Celgard 2500, denoted as PP membrane) was used as a comparison sample. The thermal stability, biodegradability, and basic electrochemical performance of the two membranes were compared for verification.

[0048] Test method for thermal stability: Place the CE film and PP film samples flat in a forced-air drying oven at 150℃ and remove them after heat treatment for 1 hour.

[0049] Biodegradability test method: CE film and PP film samples were buried in the soil respectively. The condition of the samples was observed periodically after 1 month (4 weeks).

[0050] Basic electrochemical performance testing: The dried separators (CE membrane and PP membrane) were punched into discs with a diameter of 19 mm and dried in a vacuum drying oven at 80°C for 12 hours to completely remove residual solvent. They were then transferred to a glove box filled with argon gas and assembled into LiFePO4 / lithium half-cells.

[0051] Conclusion: The PP membrane exhibited severe curling and shrinkage. The CE membrane remained flat and intact (Figure 3a), while the PP membrane showed significant shrinkage (Figure 3b), directly demonstrating that the separator of this invention possesses superior thermal dimensional stability far exceeding that of commercially available PP separators. After a 4-week soil burial experiment, the CE membrane showed significant structural disintegration until it completely disappeared (Figure 3d). In contrast, the PP membrane showed no visible changes during the same period (Figure 3c). Electrochemical impedance spectroscopy (EIS) measured the ionic conductivity of the CE membrane at room temperature to be 0.80 mS / cm, which is on par with the PP membrane (0.78 mS / cm). After 500 cycles at 0.5C, the CE membrane battery retained 95.5% of its capacity, comparable to the PP membrane battery (96.0%).

[0052] Comparative Example 1: The solvent-free replacement procedure is exactly the same as in Example 1, but step 4 (solvent replacement) is omitted, and the hydrogel after washing is directly dried in step 5.

[0053] Test results: The obtained cellulose membrane was transparent and brittle. SEM images (corresponding to Figure 4) showed a dense structure without any porous features. The porosity was <10%. This comparison strongly demonstrates that the "solvent displacement" step is indispensable for obtaining a porous structure.

[0054] Comparative Example 2: Comparison with the traditional particle pore-forming method. Referring to a certain existing technology (such as application number CN201910636225.9), cellulose was dissolved using LiCl / DMAC, sodium chloride particles smaller than 150μm were added, and after washing with water to remove salt and solvent, it was directly freeze-dried.

[0055] Test results: The obtained membrane has a relatively large pore size (approximately 30-50 μm), but the pore shape is irregular, consisting of negative pores left by particles (Figure 5). This comparison clearly shows that the present invention is fundamentally different from existing template methods in terms of pore formation mechanism, microstructure of the obtained membrane (three-dimensional nanonetwork vs. template pores), and performance indicators.

[0056] This invention provides a green preparation method for a high-porosity cellulose porous membrane based on solvent replacement and a solvent closed-loop circulation system, as shown in Figure 6. The method includes the following steps: (1) dissolving cellulose raw material in an aqueous lithium bromide solution to form a homogeneous cellulose solution; (2) molding the cellulose solution obtained in step (1) and then cooling and solidifying it to obtain a solidified cellulose gel; (3) regenerating the cellulose gel obtained in step (2) by countercurrent rinsing to induce phase separation, thereby regenerating the cellulose and simultaneously removing lithium bromide to obtain a cellulose hydrogel; (4) solvent replacement: using a low surface tension alcohol or ketone solvent, the hydrogel obtained in step (3) is fully replaced by countercurrent rinsing to replace the water in its internal pores; (5) drying the gel after replacement in step (4) to obtain the high-porosity cellulose porous membrane.

[0057] (6) Solvent recovery and recycling, specifically: (6a) Collect the lithium bromide wastewater generated in step (3), concentrate it, and return it to step (1) for recycling; (6b) Collect the water-containing alcohol or ketone solvent generated in step (4), dehydrate and purify it, and return it to step (4) for recycling; (6c) Collect the alcohol or ketone solvent dried and volatilized in step (5), and enter step (4) for recycling through the recovery pipe.

[0058] The solvent closed-loop circulation system includes: a lithium bromide solution recovery and purification unit, an alcohol solvent recovery and purification unit, a fluid delivery pipeline and a process control unit; the lithium bromide solution recovery and purification unit is used to treat the washing wastewater and return it to the dissolution process (Figure 6 bottom), and the alcohol solvent recovery and purification unit is used to treat the aqueous alcohol solution and return it to the displacement process (Figure 6 top).

[0059] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of protection of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A green method for preparing high-porosity cellulose porous membranes based on solvent displacement, characterized in that, Includes the following steps: (1) Dissolve the cellulose raw material in an aqueous lithium bromide solution to form a homogeneous cellulose solution; (2) Perform a molding operation on the cellulose solution obtained in step (1), and then cool and solidify it to obtain a solidified cellulose gel; (3) Wash the cellulose gel obtained in step (2) with countercurrent rinsing or immersion in flowing deionized water to induce phase separation, thereby regenerating the cellulose and removing lithium bromide at the same time to obtain a cellulose hydrogel; (4) Solvent replacement: Use an alcohol or ketone solvent with low surface tension to soak the hydrogel obtained in step (3) to fully replace the water in its internal pores; (5) Dry the gel after replacement in step (4) to obtain the high porosity cellulose porous membrane.

2. The method according to claim 1, characterized in that, In step (1), the mass concentration of the lithium bromide aqueous solution is 55%-65%; the dissolution temperature is 110℃-140℃.

3. The method according to claim 1, characterized in that, In step (2), the temperature of cooling and solidification is below 70°C.

4. The method according to claim 1, characterized in that, In step (4), the alcohol or ketone solvent is at least one of methanol, ethanol, isopropanol, tert-butanol or acetone.

5. The method according to claim 1, characterized in that, In step (4), the cellulose hydrogel is solvent-replaced by displacement, and finally replaced with a cellulose gel containing only alcohol or ketone solvents; in step (4), the degree of solvent displacement is based on the residual water content inside the gel after displacement being less than 1%.

6. The method according to any one of claims 1-5, characterized in that, The method further includes step (6) solvent recovery and recycling, specifically: (6a) collecting lithium bromide wastewater generated in step (3), concentrating it, and returning it to step (1) for recycling; (6b) collecting water-containing alcohol or ketone solvents generated in step (4), dehydrating and purifying them, and returning them to step (4) for recycling; (6c) collecting alcohol or ketone solvents dried and volatilized in step (5), and entering step (4) for recycling through a recovery pipe.

7. The method according to claim 6, characterized in that, In step (6a), the concentration is carried out using evaporation or membrane separation technology; in step (6b), the dehydration and purification is carried out using molecular sieve adsorption technology.

8. A high-porosity cellulose porous membrane prepared by the method according to any one of claims 1-5, characterized in that, The porosity of the porous membrane is 50-80%.

9. The application of a high-porosity cellulose porous membrane prepared by any one of claims 1-5 in the preparation of battery separators and ion filtration membranes.

10. A solvent closed-loop circulation system for implementing the method of claim 7, characterized in that, include: The system includes a lithium bromide solution recovery and purification unit, an alcohol solvent recovery and purification unit, a fluid transport pipeline, and a process control unit. The lithium bromide solution recovery and purification unit is used to treat washing wastewater and return it to the dissolution process, while the alcohol solvent recovery and purification unit is used to treat aqueous alcohol solutions and return them to the displacement process.

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