Ion regulation type cellulose composite aerogel material and preparation method thereof

Abstract

The invention provides an ion regulation type cellulose composite aerogel material and a preparation method thereof. The preparation method of the material comprises the following steps: S1, oxidizing graphite powder through a concentrated sulfuric acid / phosphoric acid mixed system to prepare graphene oxide (GO); s2, dispersing cellulose powder in an alkaline urea solution in a liquid nitrogen environment to form a homogeneous system; s3, adding GO and a cross-linking agent epichlorohydrin, and constructing cellulose / GO composite hydrogel through ice-bath stirring and thermal cross-linking; s4, soaking the hydrogel in a salt solution containing salt and anions (Cl <->, I <->), and regulating and controlling a hydrogen bond network and crystallization behavior of a cellulose molecular chain through ion-hydroxyl interaction; and S5, freeze-drying to obtain the porous aerogel. The material disclosed by the invention selectively occupies hydroxyl active sites through anions, destroys a compact crystal domain and reduces evaporation enthalpy, and an efficient and sustainable solution is provided for seawater desalination and sewage purification.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional materials and new energy, and particularly relates to a cellulose composite aerogel material based on an ion regulation strategy and a preparation method thereof, which is particularly suitable for solar-driven interfacial water evaporation, seawater desalination, and high-salt wastewater treatment. Background Art

[0002] As a natural renewable polymer, cellulose is regarded as an ideal evaporation substrate material due to its rich hydroxyl groups and three-dimensional porous structure. However, during the processing and drying of cellulose, excessive intermolecular hydrogen bonding will form dense crystalline domains, resulting in a decrease in porosity and an increase in water transport resistance, severely limiting the evaporation efficiency. Existing technologies such as directional freezing, 3D printing, or surface modification can partially improve the water transport ability of cellulose, but there are problems such as complex processes, high costs, or poor structural stability. In addition, although synthetic polymer hydrogels can reduce the evaporation enthalpy by regulating hydrophilic groups, their non-degradability and high-energy-consuming synthesis processes are contrary to the concept of sustainable development.

[0003] In recent years, the ion-specific effect (Hofmeister effect) has been proven to affect the hydrophilicity and network structure of materials by regulating intermolecular forces. For example, strongly hydrated anions can induce the aggregation of cellulose chains, while weakly hydrated anions disrupt the hydrogen bond network by occupying hydroxyl sites. However, existing research has mainly focused on the dissolution mechanism of cellulose, and the influence mechanism of ion regulation on the crystallization behavior and evaporation performance of cellulose evaporation materials is still unclear. In particular, there is still a lack of effective strategies for balancing the depolymerization of crystalline domains and structural stability.

[0004] Based on this, the present invention proposes to prepare a hierarchically porous cellulose composite aerogel by "salt-in" anions selectively occupying the active sites of cellulose hydroxyl groups and combining the photothermal synergistic effect of graphene oxide. This method regulates the hydrogen bond network at the molecular scale, enhances the water transport ability while reducing the evaporation enthalpy, and endows the material with excellent salt resistance and environmental friendliness, providing a new idea for the development of high-performance solar evaporators. Summary of the Invention

[0005] Aiming at the problems of low water transport efficiency, complex synthesis process, and poor environmental compatibility of existing cellulose-based evaporation materials, the present invention proposes a cellulose composite aerogel material based on an ion regulation strategy and a preparation method thereof. By soaking in different ionic liquids, the gel pores and water activation effect are regulated to achieve improved evaporation performance. The technical solution of the present invention is realized as follows:

[0006] A preparation method of an ion-regulated cellulose composite aerogel material, the specific steps include:

[0007] S1. Preparation of graphene oxide: Graphite powder is oxidized in a mixed solution of concentrated sulfuric acid and phosphoric acid with a volume ratio of 9:1. After the reaction under ice bath, hydrogen peroxide is added to terminate the reaction, and then it is centrifuged, washed until neutral, and freeze-dried to obtain graphene oxide (GO).

[0008] S2. Pre-dispersion of cellulose: Cellulose powder is dissolved in an alkaline urea system under liquid nitrogen environment to form a homogeneous cellulose solution with a mass concentration of 3.75 wt%.

[0009] S3. Construction of composite gel: GO powder and cross-linking agent epichlorohydrin (ECH) are added to the solution in step S2. After stirring under ice bath, it is poured into a mold and thermally cross-linked in a 60°C water bath to form a cellulose / GO composite hydrogel.

[0010] S4. Ion-induced modification: The hydrogel is immersed in a salt solution containing anions (Cl - , I - ) to regulate the hydrogen bond network and crystallization behavior of cellulose molecular chains.

[0011] S5. Freeze-drying and forming: After washing to remove residual ions, it is freeze-dried to obtain a porous cellulose composite aerogel.

[0012] Furthermore, in step S1, the mass fraction of the concentrated sulfuric acid is ≥98%, the mass-volume ratio of graphite powder to the mixed acid is 1 g:10 mL, and the oxidation reaction conditions are 50°C for 12 hours.

[0013] Furthermore, in step S2, the alkaline urea system is composed of sodium hydroxide, urea, and water with a mass ratio of 7:12:81, and the dissolution temperature is -20°C (liquid nitrogen environment).

[0014] Furthermore, in step S3, the addition amount of GO is 0.8 - 1.2% of the cellulose mass, the addition amount of the cross-linking agent epichlorohydrin is 0.8 mL for 10 mL of the solution, and the thermal cross-linking time is 4 hours.

[0015] Furthermore, in step S4, the salt solution is 1 M NaCl solution, NaI solution, and the soaking time is 24 hours, which is used to selectively occupy the active sites of cellulose hydroxyl groups to reduce the crystallinity.

[0016] Furthermore, in step S5, the freeze-drying conditions are pre-freezing at -20°C for 8 hours and vacuum drying for 24 hours.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] In the present invention, by introducing "salt-in" anions to selectively occupy the active sites of cellulose hydroxyl groups, the dense hydrogen bond network is effectively destroyed, the evaporation enthalpy is significantly reduced, and high-efficient evaporation performance is achieved. Description of the Drawings

[0019] Figure 1 SEM electron micrographs of the corresponding aerogels for Examples 1-2.

[0020] Figure 2 XRD patterns of Examples 1-2.

[0021] Figure 3 DSC curves of Examples 1-2.

[0022] Figure 4 Infrared spectra of Examples 1-2.

[0023] Figure 5 Evaporation rate graphs of Examples 1-2.

[0024] Figure 6 Evaporation rate graphs of the comparative examples.

[0025] Figure 7 Ion concentration graphs of Example 2. Detailed Description of the Invention

[0026] To better understand the technical content of the present invention, specific examples are provided below to further illustrate the present invention.

[0027] Unless otherwise specified, the experimental methods used in the examples of the present invention are all conventional methods.

[0028] Unless otherwise specified, the materials, reagents, etc. used in the examples of the present invention can all be obtained from commercial sources.

[0029] Example 1

[0030] Example 1 is a preparation method of Cl - modified cellulose aerogel (CHG), and the specific steps include:

[0031] S1. Add 1.0 g of graphite powder to a mixed solution of 90 mL of concentrated sulfuric acid (98%) and 10 mL of phosphoric acid (85%), and carry out an oxidation reaction at 50 °C for 12 hours. After termination with H 2 O 2 centrifuge and freeze-dry to obtain GO powder.

[0032] S2. Mix 7.0 g of NaOH, 12.0 g of urea and 81.0 g of deionized water to prepare an alkaline urea pre-cooled solution (-20 °C), add 3.75 g of cellulose powder and stir in a liquid nitrogen environment for 2 hours to dissolve, forming a homogeneous cellulose solution

[0033] S3. Add 0.03 g of GO (0.8% of the cellulose mass) and 0.8 mL of epichlorohydrin (ECH) to the solution. After stirring in an ice bath for 150 minutes, pour it into a mold and thermally crosslink at 60 °C for 4 hours to form a composite hydrogel;

[0034] S4. Composite gel construction: Add GO powder and the crosslinking agent epichlorohydrin (ECH) to the solution in step S2. After stirring in an ice bath, pour it into a mold and thermally crosslink in a 60 °C water bath to form a cellulose / GO composite hydrogel;

[0035] S5. Immerse the hydrogel in 1 M NaCl solution (58.44 g of NaCl dissolved in 1 L of deionized water), soak at room temperature for 24 hours for ionic modification; after washing, freeze-dry at -50 °C to obtain the CHG-Cl - aerogel.

[0036] S6. Immerse the gel in deionized water for cleaning, pre-freeze at -20 °C for 8 hours and vacuum dry for 24 hours to obtain the CHG aerogel.

[0037] Example 2

[0038] Prepare I on the basis of Example 1 - Modified fiber aerogel

[0039] S1. Add 1.0 g of graphite powder to a mixture of 90 mL of concentrated sulfuric acid (98%) and 10 mL of phosphoric acid (85%), carry out an oxidation reaction at 50 °C for 12 hours, and after termination with H 2 O 2 centrifuge and freeze-dry to obtain GO powder.

[0040] S2. Mix 7.0 g of NaOH, 12.0 g of urea and 81.0 g of deionized water to prepare an alkaline urea pre-cooled solution (-20 °C), add 3.75 g of cellulose powder and stir in a liquid nitrogen environment for 2 hours to dissolve, forming a homogeneous cellulose solution

[0041] S3. Add 0.03 g of GO (0.8% of the cellulose mass) and 0.8 mL of epichlorohydrin (ECH) to the solution. After stirring in an ice bath for 150 minutes, pour it into a mold and thermally crosslink at 60 °C for 4 hours to form a composite hydrogel;

[0042] S4. Composite gel construction: Add GO powder and the crosslinking agent epichlorohydrin (ECH) to the solution in step S2. After stirring in an ice bath, pour it into a mold and thermally crosslink in a 60 °C water bath to form a cellulose / GO composite hydrogel;

[0043] S5. Immerse the hydrogel in 1 M NaI solution (149.89 g of NaI dissolved in 1 L of deionized water) for 24 hours, and freeze-dry to obtain the CHG-I - aerogel.

[0044] S6. After immersing the gel in deionized water for cleaning, it was pre-frozen at -20 °C for 8 hours and vacuum dried for 24 hours to obtain CHG aerogel.

[0045] 1. The Cl - modified cellulose aerogel (CHG-Cl - ) and the I - modified fiber aerogel (CHG-I - ) prepared as described above were observed for their microscopic morphology by SEM, see Figure 1 . As can be seen from Figure 1 , the pore size of the I - modified fiber aerogel is larger than that of the Cl - modified cellulose aerogel.

[0046] 2. The Cl - modified cellulose aerogel (CHG-Cl - ) and the I - modified fiber aerogel (CHG-I - ) prepared as described above were analyzed by XRD. See Figure 2 . As shown in Figure 2 , the characteristic peaks that appeared did not change after soaking in different solutions and still belonged to the cellulose II crystal plane.

[0047] 3. The Cl - modified cellulose aerogel (CHG-Cl - ) and the I - modified fiber aerogel (CHG-I - ) and water were analyzed by DSC. See Figure 3 . As shown in Figure 3 , it can be seen that the evaporation enthalpy value is lower than that of water, which indicates that anions with specific ionic effects can regulate the water activation process of cellulose aerogel, increase the bound water content and reduce the evaporation enthalpy.

[0048] 4. The I - modified fiber aerogel (CHG-I - ) prepared in Example 2 was analyzed by Fourier transform infrared spectroscopy. As shown in Figure 4 , obvious graphene oxide functional groups can be seen from Figure 4 . In addition, graphene oxide functional groups were also found in CHG-I - , indicating that graphene oxide has been successfully integrated into CHG-I - .

[0049] 5. The Cl -Modified cellulose aerogel (CHG-Cl - ) and the water evaporation analysis was carried out on the modified fiber aerogel (CHG-I - ) prepared through I - . See Figure 5 . As Figure 5 shown, the evaporation rate of the gel with I - is greater than that of the gel with Cl - . This is mainly because the evaporation enthalpy of the gel with I - is lower. Under the same energy input, only a smaller amount of energy is required to achieve the escape of water molecules.

[0050] Comparative example

[0051] The preparation method of the unmodified cellulose aerogel (CHG) in this example specifically includes the following steps:

[0052] S1. Add 1.0 g of graphite powder to a mixed solution of 90 mL of concentrated sulfuric acid (98%) and 10 mL of phosphoric acid (85%), carry out an oxidation reaction at 50 °C for 12 hours, and terminate with H 2 O 2 , and then centrifuge and freeze-dry to obtain GO powder.

[0053] S2. Mix 7.0 g of NaOH, 12.0 g of urea and 81.0 g of deionized water to prepare an alkaline urea precooled solution (-20 °C), add 3.75 g of cellulose powder, and stir in a liquid nitrogen environment for 2 hours to dissolve, forming a homogeneous cellulose solution

[0054] S3. Add 0.03 g of GO (accounting for 0.8% of the cellulose mass) and 0.8 mL of epichlorohydrin (ECH) to the solution, stir in an ice bath for 150 minutes, then inject into a mold, and carry out thermal cross-linking at 60 °C for 4 hours to form a composite hydrogel;

[0055] S4. Composite gel construction: Add GO powder and cross-linking agent epichlorohydrin (ECH) to the solution in step S2, stir in an ice bath, then pour into a mold, and carry out thermal cross-linking in a 60 °C water bath to form a cellulose / GO composite hydrogel;

[0056] S5. Immerse the gel in deionized water for cleaning, then pre-freeze at -20 °C for 8 hours and vacuum dry for 24 hours to obtain CHG aerogel.

[0057] Refer to the preparation of the comparative example Figure 6 . As Figure 6 shown, the evaporation rate of the unmodified cellulose gel is lower than that of the gel modified by the example. This indicates that the unmodified gel does not have anions with specific ionic effects and it is difficult to regulate the water activation process of cellulose aerogel.

[0058] Application example

[0059] The photothermal interfacial evaporation performance of the prepared I - modified fiber aerogel (CHG-I - ) was evaluated. See Figure 7 , as Figure 7 shown, compared with natural seawater, the ion concentration in the generated condensate water was significantly reduced, and the desalination efficiency was as high as 99.95%. It meets the WHO drinking water standard.

[0060] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing an ion-regulated cellulose composite aerogel, characterized in that: The specific steps include: S1. Preparation of graphene oxide: oxidize graphite powder in a mixed solution of concentrated sulfuric acid and phosphoric acid in a volume ratio of 9:1, add hydrogen peroxide to terminate the reaction after ice bath reaction, wash by centrifugation until neutral, and freeze-dry to obtain graphene oxide; S2, cellulose pre-dispersion: dissolving cellulose powder in an alkaline urea system under a liquid nitrogen environment to form a homogeneous cellulose solution with a mass concentration of 3.75wt%; S3, composite gel construction: add GO powder and cross-linking agent epichlorohydrin to the solution of step S2, stir in an ice bath and pour into a mold, and heat cross-link in a 60°C water bath to form a cellulose / GO composite hydrogel; S4, ion-induced modification: immersing the hydrogel in a salt solution containing saline anions (Cl-, I-) to regulate the hydrogen bond network and crystallization behavior of the cellulose molecular chain; S5. Freeze-drying molding: After washing to remove residual ions, freeze-drying is performed to obtain a porous cellulose composite aerogel.

2. The preparation method according to claim 1, characterized in that In step S1, the mass fraction of the concentrated sulfuric acid is ≥98%, the mass volume ratio of graphite powder to mixed acid is 1 g:10 mL, and the oxidation reaction condition is 50° C. for 12 hours.

3. The preparation method according to claim 1, characterized in that: In step S2, the alkaline urea system is composed of sodium hydroxide, urea and water in a mass ratio of 7g:12g:81g, and the dissolution temperature is -20°C.

4. The preparation method according to claim 1, characterized in that: In step S3, the amount of GO added is 0.8-1.2% of the mass of cellulose, the amount of cross-linking agent epichlorohydrin added is 0.8 mL for 10 mL of solution, and the thermal cross-linking time is 5 hours.

5. The preparation method according to claim 1, characterized in that: In step S4, the salt solution is a 1M NaCl solution or a NaI solution, and the soaking time is 24 hours, which is used to selectively occupy the hydroxyl active sites of cellulose.

6. The preparation method according to claim 1, characterized in that: In step S5, the freeze-drying conditions are pre-freezing at -20°C for 8 hours and vacuum drying for 24 hours.

7. An ion-regulated cellulose composite aerogel prepared by the method of any one of claims 1 to 6.

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