CN106867019A - One-pot method for preparing SiO2Method for producing cellulose composite aerogel material - Google Patents

Abstract

The invention relates to a one-pot method for preparing SiO ₂ -cellulose composite airgel materials, which uses cellulose as a raw material and dissolves the cellulose in a green alkali urine solvent; uses inorganic sodium silicate as a silicon source, and obtains fibers by stirring and dispersing The regenerated cellulose wet gel containing silicon source was obtained by combining the sol-gel method with the aqueous solution uniformly mixed with sodium silicate; the cellulose gel was used as the skeleton, and the inorganic acid was used as the reaction solvent to induce the formation of SiO ₂ , and finally the CO ₂ Drying by supercritical drying process to obtain SiO ₂ -cellulose organic-inorganic composite airgel. The process of the present invention is simple and fast, high in feasibility and environmental protection, effectively overcomes the problems of poor dispersion of _SiO2 in the porous cellulose network structure in the in-situ impregnation method, low composite content and poor composite stability, and the prepared inorganic-organic composite gas Gel has low thermal conductivity and good thermal insulation performance, and has good application prospects in low-temperature cold insulation materials, medium and low temperature thermal insulation materials, and adsorption materials.

Description

One-pot method for preparing SiO2-cellulose composite airgel materials

technical field

The invention belongs to the field of preparation of polymer nano functional materials, and relates to a method for preparing SiO ₂ -cellulose composite airgel materials in one pot.

Background technique

Cellulose airgel is a new type of new natural biomass material developed in recent years. Cellulose is the green renewable resource with the largest reserves on the earth. Cellulose airgel has both the characteristics of renewable natural polymers and many advantages of traditional high-porosity nano-inorganic porous materials. Cellulose airgel has the characteristics of good toughness and easy processing. Therefore, cellulose aerogel has become a new material with broad application prospects and high development value. Electrode materials for carbon batteries and fuel cells have great potential.

Cellulose airgel has many excellent properties, but the limitations of polysaccharide airgel itself, such as poor stability and poor flame retardancy, limit its development to a certain extent. Although traditional SiO ₂ aerogels have excellent properties, their fragility is difficult to overcome. The cellulose-SiO ₂ composite aerogel not only has the characteristics of the two aerogels, but also exhibits many new excellent properties.

Conventional SiO ₂ -cellulose composite aerogels are usually impregnated with wet cellulose membranes into tetraethyl orthosilicate/methyl orthosilicate, silica airgel is formed in the micropores of the cellulose matrix, and then Drying in supercritical CO ₂ , etc., however, this method will quickly gel silica on the outer surface of the cellulose, thereby blocking the pore openings and hindering the penetration of silica into the cellulose matrix, resulting in an urgent decrease in porosity . However, it is not advisable to directly mix the cellulose solution with tetraethyl orthosilicate, the alkali-soluble cellulose solution is a metastable system, and directly mixing with the silane hydrolyzate will cause the cellulose to immediately solidify and the silica sol to phase separate.

Aqueous sodium silicate is a basic silicate, and the pH of the saturated solution is about 14; similarly, the alkali-soluble cellulose aqueous sodium silicate aqueous solution with a pH of 14 is used as the starting solution, and the one-pot method can be directly mixed to form a uniform and stable solution. Mix the solution, the self-association of cellulose chains in the alcohol bath will cause gelation to form type II cellulose gel, the sodium silicate solution is dispersed in the cellulose network structure, and then the coagulated cellulose is placed in an acidic Sodium silicate can be converted to SiO ₂ when in a water bath.

Contents of the invention

The purpose of the present invention is to provide a method for preparing SiO ₂ -cellulose composite airgel materials by pot method in view of the limitations in the preparation process of the existing SiO ₂ -cellulose composite airgel, the method is feasible and simple, by adopting The one-pot method directly disperses the inorganic silicon source evenly into the three-dimensional network structure of the cellulose gel, which avoids the phenomenon of channel blockage and phase separation, and can also improve the mechanical properties of SiO ₂ -cellulose airgel to a certain extent and thermal insulation effect.

The technical scheme of the present invention is: a method for preparing SiO ₂ -cellulose composite airgel material by one-pot method, and its specific steps are as follows:

(1) Configuration of cellulose solution

Disperse the cellulose fiber into the mixed aqueous solution of alkali urea under the action of stirring, seal it and pre-cool it to -20~-5°C, take it out, stir and dissolve the cellulose with an electric mixer to obtain a viscous solution, and then centrifuge to remove air bubbles to obtain a clear and transparent The cellulose solution; wherein the mass fractions of cellulose, alkali, urea and deionized water in the cellulose solution are (2%-6%), (4%-20%), (10%-20%), ( 54%-84%);

(2) Configuration of inorganic silicon source-cellulose mixed solution

Dissolving the inorganic silicon source under agitation, to obtain a mass fraction of 1%-7% inorganic silicon source aqueous solution; according to the volume ratio of the cellulose aqueous solution and the inorganic silicon source aqueous solution is 1: (0.1-1), the inorganic silicon source aqueous solution is added to In the cellulose aqueous solution, after stirring and mixing, ultrasonic vibration removes air bubbles to form a uniformly mixed inorganic silicon source-cellulose mixed solution;

(3) Preparation of inorganic silicon source-cellulose wet gel

Pour the uniformly mixed inorganic silicon source-cellulose mixed solution into a mold, place it in a coagulation bath for regeneration, and let stand to obtain a cellulose wet gel containing an inorganic silicon source;

(4) Preparation of silica-cellulose wet gel

Transfer the cellulose wet gel containing the inorganic silicon source to a dilute acid solution, and induce the inorganic silicon source to form silica in the pores of the cellulose gel after a neutralization reaction;

(5) Preparation of composite airgel

Rinse the silica-cellulose wet gel with distilled water to make the pH of the solution 6-8; then transfer the silica-cellulose hydrogel to an organic solvent for solvent placement and aging to obtain silica-cellulose alcohol Gel; then transfer to _CO2 supercritical drying device for drying treatment, and finally obtain silica-cellulose composite aerogel.

Preferably, the cellulose fiber described in step (1) is one of cotton pulp, short-staple cotton or microcrystalline cellulose; the alkali in the described alkali urea mixed aqueous solution is sodium hydroxide or lithium hydroxide One; urea is urea or thiourea.

In the preferred step (1), the rotating speed of the electric mixer is 1000-2000rpm, and the stirring time is 4-12min; the rotating speed of the centrifuge is 8000-10000rpm, and the centrifuging time is 5-10min.

Preferably, the inorganic silicon source described in step (2) is sodium silicate pentahydrate powder, sodium silicate nonahydrate powder, anhydrous sodium silicate or alkaline water glass.

The coagulation bath described in the preferred step (3) is methanol, ethanol, acetone or dilute sulfuric acid. The standing time in the preferred step (3) is 5-360min.

Preferably, the dilute acid solution described in step (4) is aqueous hydrochloric acid, nitric acid, sulfuric acid or acetic acid; the concentration of the dilute acid solution is 0.5-1mol/L.

Preferably, the organic solvent described in step (5) is one of methanol, ethanol or acetone or a mixture thereof.

Preferably, the carbon dioxide supercritical drying method described in step (5) uses carbon dioxide as the drying medium, the reaction temperature is 40-70° C., the pressure in the autoclave is 8-12 MPa, and the drying time is 10-20 hours.

Preferably, the silica-cellulose composite airgel prepared in step (5) has an apparent density of 0.092-0.143 g/cm ³ , a specific surface area of 227.41-264.93 m ² /g, and a thermal conductivity of 0.029-0.038 W/m·K.

Beneficial effect:

The method of the present invention and the organic-inorganic composite SiO ₂ -cellulose airgel material prepared by the one-pot method have the following characteristics:

(1) The process is simple and effective. The preparation method overcomes the problem of pore blockage in the SiO ₂ -cellulose airgel material in the impregnation method and the phase separation problem of the organic silicon source and the alkali-soluble cellulose solution. In the impregnation method, the silica sol fills the porous cellulose gel by penetrating into the three-dimensional cellulose matrix. This process has certain limitations because it is a diffusion process, so the distribution of silica in the cellulose matrix is not uniform, it is susceptible to the preparation of the cellulose gel, and the impregnation process takes a long time. However, directly mixing the organic silicon source and the aqueous solution of alkali-soluble cellulose is in a metastable dissolution system because the aqueous solution of alkali-soluble cellulose is not really dissolved. Water exposes the inter- and intra-molecular hydrogen bonds of cellulose, resulting in rapid gelation and phase separation of cellulose. However, sodium silicate, which is also alkaline, is selected as the silicon source. After dissolving, it is directly mixed into the alkali-lyzed cellulose solution without destroying the metastable dissolution system. At the same time, it also inhibits the occurrence of phase separation, and can form a uniformly mixed stable solution. . After alcohol bath gelation, under the action of acid, the sodium silicate is converted to the direction of producing silicon dioxide and sodium salt, and finally SiO ₂ -cellulose composite airgel is obtained by supercritical drying.

(2) The raw material is cheap and has a wide range of sources. The cellulose and sodium silicate (water glass) mainly selected are cheap and easy-to-get industrial raw materials, which greatly reduce the manufacturing cost and are easy to realize large-scale production. And it has good environmental protection effect. At the same time, all the cellulose dissolving systems selected are green dissolving, and only need materials such as sodium hydroxide and urea to dissolve cellulose in a low temperature environment, which is simple and effective.

(3) Compared with other airgel insulation materials, the SiO ₂ -cellulose composite airgel is prepared by the one-pot mixing method. The three-dimensional network structure of the cellulose gel is used as the generation template, and the sodium silicate is filled in the cellulose In the pores of the cellulose, SiO ₂ particles are generated under the catalysis of the acidic solution, and SiO ₂ condenses into larger spherical particles to form agglomerates attached to the nanofibrils of cellulose, avoiding the appearance of free SiO ₂ particles, and the prepared The surface of the material is smooth, the thickness is controllable, and the formability is good. The microscopic appearance of the prepared SiO ₂ -cellulose composite airgel is shown by the scanning electron microscope. And the prepared SiO ₂ -cellulose composite airgel still maintains extremely high porosity and relatively uniform pore structure.

Description of drawings

Fig. 1 is the sample diagram of SiO ₂ -cellulose composite airgel material prepared in Example 1;

Fig. 2 is the X-ray diffraction spectrogram of the SiO ₂ -cellulose composite airgel material prepared in Example 2;

Fig. 3 is the scanning electron micrograph of the SiO ₂ -cellulose composite airgel material prepared in Example 2;

Fig. 4 is the nitrogen adsorption-desorption curve of the SiO ₂ -cellulose composite airgel prepared in Example 3.

detailed description

Example 1

1. Stir and disperse 2g of microcrystalline cellulose into the alkali urea solution (4.0gLiOH/10gUrea/84gH ₂ O), transfer to the refrigerator to pre-cool at -5°C after dispersion, and use the motor to stir at 2000rpm immediately after taking it out Dissolve the cellulose for 4 minutes to obtain a viscous solution, then immediately centrifuge at 8000 rpm for 10 minutes to remove air bubbles to obtain a clear and transparent cellulose solution with a solvent mass fraction of 2%; weigh 16.3g of Na ₂ SiO ₃ ·9H ₂ O magnetically stirred and dissolved in 83.7g of distilled water to obtain a sodium silicate aqueous solution with a mass fraction of 7%. The cellulose solution and the sodium silicate solution were mechanically stirred and mixed at a volume ratio of 10:1, and then ultrasonically oscillated to remove air bubbles to form a uniform Mixed sodium silicate-cellulose mixed solution. Then pour the uniformly mixed sodium silicate-cellulose mixed solution into the mold and place it in a methanol coagulation bath for regeneration. After standing for 360 minutes, a cellulose wet gel containing sodium silicate is obtained. After aging, remove the cellulose wet gel containing sodium silicate and transfer it to a dilute sulfuric acid solution with a volume of 200ML and a concentration of 0.5mol/L, and induce silica in the pores of the cellulose gel after a neutralization reaction. sodium phosphate to generate silica, and then the silica-cellulose composite wet gel was rinsed with distilled water to remove excess acid and generated salt, and the pH of the solution was 7 at this time. Then the silica-cellulose hydrogel was transferred to an ethanol solvent for solvent replacement and aging, a total of 8 replacements, and finally the silica-cellulose alcohol gel was obtained, and the silica-cellulose alcohol gel was placed in The drying process is carried out in a _CO2 supercritical drying device, the drying temperature is 40°C, the pressure in the autoclave is 8MPa, and the drying time is 20h. After supercritical CO ₂ drying, organic-inorganic SiO ₂ -cellulose aerogels are formed. The density is 0.092/cm ³ , the specific surface area is 227.41 m ² /g, and the thermal conductivity coefficient is 0.032 W/m·K. Accompanying drawing 1 is a sample diagram of the SiO ₂ -cellulose composite airgel material prepared in Example 1. It can be seen from the figure that the composite airgel and the pure cellulose airgel have the same milky white color, the surface of the material is smooth, there is no powder drop, and it also has good mechanical strength.

Example 2

Stir and disperse 6g of cotton pulp cellulose into alkali urea solution (4gLiOH/10gThourea/80gH ₂ O), transfer to the refrigerator to pre-cool at -20°C after dispersion, and immediately use a motor to stir at 2000rpm for 4min to dissolve The viscous solution was obtained from the cellulose, and then immediately centrifuged at 10,000 rpm for 5 minutes to remove air bubbles to obtain a clear and transparent cellulose solution with a solvent mass fraction of 6%; weigh 1.74g of Na ₂ SiO ₃ 5H ₂ O magnetic Stir and dissolve into 98.3g of distilled water to obtain a sodium silicate aqueous solution with a mass fraction of 1%. The cellulose solution and the sodium silicate solution are mechanically stirred and mixed at a volume ratio of 1:1, and ultrasonic vibration is used to remove air bubbles to form a uniformly mixed silicate solution. sodium bicarbonate-cellulose mixed solution. Then pour the uniformly mixed sodium silicate-cellulose mixed solution into the mold and place it in an ethanol coagulation bath for regeneration, and after standing for 60 minutes, a cellulose wet gel containing sodium silicate is obtained. After aging, remove the cellulose wet gel containing sodium silicate and transfer it to a dilute hydrochloric acid solution with a volume of 100ML and a concentration of 1mol/L, and induce silicic acid in the pores of the cellulose gel after a neutralization reaction occurs. Sodium generates silica, and then the silica-cellulose composite wet gel is rinsed with de-distilled water to remove excess acid and generated salt, and the pH of the washing solution is now 7. Then the silica-cellulose hydrogel was transferred to a methanol solvent for solvent replacement and aging, a total of 5 replacements, and finally the silica-cellulose alcohol gel was obtained, and the silica-cellulose alcohol gel was placed in The drying process is carried out in a _CO2 supercritical drying device, the drying temperature is 40°C, the pressure in the autoclave is 8MPa, and the drying time is 20h. After supercritical CO ₂ drying, organic-inorganic SiO ₂ -cellulose aerogels are formed. The density is 0.143/cm ³ , the specific surface area is 243.46 m ² /g, and the thermal conductivity coefficient is 0.032 W/m·K. Figure 2 is a scanning electron microscope picture of SiO ₂ -cellulose composite airgel material, SiO ₂ is condensed into agglomerates of larger spherical particles, attached to the three-dimensional network structure of cellulose gel (marked in the figure ), SiO ₂ is formed in the pore structure of cellulose, and no free SiO ₂ particles appear. Figure 3 is the cross-sectional energy spectrum of SiO ₂ -cellulose composite airgel. EDS energy spectrum analysis shows that the Si peak is located at 1.73KeV, and the Na peak is at 1.04KeV, which is mainly due to the sodium salt produced during the transformation process of SiO ₂ .

Example 3

Stir and disperse 3g of linter cellulose into the alkali urea solution (4.6gNaOH/15gUrea/77.4gH ₂ O), transfer to the refrigerator to pre-cool at -15°C after dispersion, and use the motor immediately after taking it out under the condition of 1000rpm Stir for 12 minutes to dissolve the cellulose to obtain a viscous solution, then immediately centrifuge at 8000rpm for 10 minutes to remove air bubbles to obtain a clear and transparent cellulose solution with a solvent mass fraction of about 3%; weigh 3g of anhydrous Na ₂ SiO ₃ Dissolve in 97ml of distilled water with magnetic stirring to obtain a sodium silicate aqueous solution with a mass fraction of 3%. The cellulose solution and sodium silicate solution are mechanically stirred and mixed at a volume ratio of 2:1, and ultrasonically oscillated to remove air bubbles. A uniformly mixed sodium silicate-cellulose mixed solution is formed. Then pour the uniformly mixed sodium silicate-cellulose mixed solution into the mold and place it in an acetone coagulation bath for regeneration. After standing for 5 minutes, a cellulose wet gel containing sodium silicate is obtained. After aging, the cellulose wet gel wet gel containing sodium silicate is removed from the mold and then transferred to a volume of 200ML and the concentration is 1mol/L in dilute nitric acid solution. After the neutralization reaction occurs, in the cellulose gel hole The sodium silicate is induced to generate silica, and then the silica-cellulose composite wet gel is rinsed with de-distilled water to remove excess acid and generated salt, and the pH of the washing solution is 6. Then the silica-cellulose hydrogel was transferred to acetone solvent for solvent replacement and aging, a total of 10 replacements, and finally the silica-cellulose alcohol gel was obtained, and the silica-cellulose alcohol gel was placed in The drying process is carried out in a _CO2 supercritical drying device, the drying temperature is 70°C, the pressure in the autoclave is 12MPa, and the drying time is 10h. After supercritical CO ₂ drying, organic-inorganic SiO ₂ -cellulose aerogels are formed. The density is 0.109 g/cm ³ , the specific surface area is 264.93 m ² /g, and the thermal conductivity coefficient is 0.029 W/m·K. Fig. 4 is the nitrogen adsorption-desorption curve of the prepared SiO ₂ -cellulose composite airgel, which belongs to type IV in the IUPAC classification and conforms to the inherent essential characteristics of porous materials.

Example 4

Stir and disperse 6g of linter cellulose into the alkali urea solution (20gNaOH/20gUrea/54gH ₂ O), transfer to the refrigerator to pre-cool at -15°C after dispersion, and immediately use the motor to stir at 1000rpm for 5min Dissolve the cellulose to obtain a viscous solution, then immediately centrifuge at 8000rpm for 5 minutes to remove air bubbles to obtain a clear and transparent cellulose solution with a solvent mass fraction of about 3%; weigh 3g of alkaline water glass and magnetically stir to dissolve Add 97g of distilled water to obtain a sodium silicate aqueous solution with a mass fraction of about 3%. After the cellulose solution and the sodium silicate solution are mechanically stirred and mixed at a volume ratio of 1:1, the bubbles are removed by ultrasonic vibration to form a uniformly mixed silicic acid Sodium-cellulose mixed solution. Then pour the uniformly mixed sodium silicate-cellulose mixed solution into the mold and place it in a dilute sulfuric acid coagulation bath for regeneration, and after standing for 5 minutes, a cellulose wet gel containing sodium silicate is obtained. After aging, remove the cellulose wet gel containing sodium silicate from the mold and transfer it to a dilute acetic acid solution with a volume of 200ML and a concentration of 1mol/L, and induce silicic acid in the pores of the cellulose gel after a neutralization reaction. Sodium generates silica, and then the silica-cellulose composite wet gel is rinsed with de-distilled water to remove excess acid and generated salt, and the pH of the washing solution is 8. Then the silica-cellulose hydrogel was transferred to an ethanol solvent for solvent replacement and aging, a total of 7 replacements, and finally the silica-cellulose alcohol gel was obtained, and the silica-cellulose alcohol gel was placed in The drying process is carried out in a _CO2 supercritical drying device, the drying temperature is 40°C, the pressure in the autoclave is 8MPa, and the drying time is 20h. After supercritical CO ₂ drying, organic-inorganic SiO ₂ -cellulose aerogels are formed. The density is 0.128/cm ³ , the specific surface area is 238.56 m ² /g, and the thermal conductivity coefficient is 0.033 W/m·K.

Claims (10)

1. One-pot method prepares the method for SiO ₂ -cellulose composite airgel material, and its specific steps are as follows: (1) Configuration of cellulose solution Disperse the cellulose fiber into the mixed aqueous solution of alkali urea under the action of stirring, seal it and pre-cool it to -20~-5°C, take it out, stir and dissolve the cellulose with an electric mixer to obtain a viscous solution, and then centrifuge to remove air bubbles to obtain a clear and transparent The cellulose solution; wherein the mass fractions of cellulose, alkali, urea and deionized water in the cellulose solution are (2%-6%), (4%-20%), (10%-20%), ( 54%-84%); (2) Configuration of inorganic silicon source-cellulose mixed solution Dissolving the inorganic silicon source under agitation, to obtain a mass fraction of 1%-7% inorganic silicon source aqueous solution; according to the volume ratio of the cellulose aqueous solution and the inorganic silicon source aqueous solution is 1: (0.1-1), the inorganic silicon source aqueous solution is added to In the cellulose aqueous solution, after stirring and mixing, ultrasonic vibration removes air bubbles to form a uniformly mixed inorganic silicon source-cellulose mixed solution; (3) Preparation of inorganic silicon source-cellulose wet gel Pour the uniformly mixed inorganic silicon source-cellulose mixed solution into a mold, place it in a coagulation bath for regeneration, and let stand to obtain a cellulose wet gel containing an inorganic silicon source; (4) Preparation of silica-cellulose wet gel Transfer the cellulose wet gel containing the inorganic silicon source to a dilute acid solution, and induce the inorganic silicon source to form silica in the pores of the cellulose gel after a neutralization reaction; (5) Preparation of composite airgel Rinse the silica-cellulose wet gel with distilled water to make the pH of the solution 6-8; then transfer the silica-cellulose hydrogel to an organic solvent for solvent placement and aging to obtain silica-cellulose alcohol Gel; then transfer to _CO2 supercritical drying device for drying treatment, and finally obtain silica-cellulose composite aerogel. 2. method according to claim 1, is characterized in that the cellulose fiber described in step (1) is the one in cotton pulp, short-staple cotton or microcrystalline cellulose; Described alkali urea mixed aqueous solution The alkali is one of sodium hydroxide or lithium hydroxide; urea is urea or thiourea. 3. method according to claim 1, it is characterized in that the rotating speed of electric mixer is 1000-2000rpm in the step (1), and stirring time is 4-12min; The rotating speed of centrifuge is 8000-10000rpm, and centrifugal time is 5-10min . 4. The method according to claim 1, characterized in that the inorganic silicon source in step (2) is sodium silicate pentahydrate powder, sodium silicate nonahydrate powder, anhydrous sodium silicate or alkaline water glass. 5. The method according to claim 1, characterized in that the coagulation bath described in step (3) is methanol, ethanol, acetone or dilute sulfuric acid. 6. The method according to claim 1, characterized in that the standing time in step (3) is 5-360min. 7. The method according to claim 1, characterized in that the dilute acid solution described in step (4) is hydrochloric acid, nitric acid, sulfuric acid or acetic acid aqueous solution; the concentration of the dilute acid solution is 0.5-1mol/L. 8. The method according to claim 1, characterized in that the organic solvent described in step (5) is one of methanol, ethanol or acetone or a mixture thereof. 9. The method according to claim 1, characterized in that the carbon dioxide supercritical drying method described in step (5) uses carbon dioxide as the drying medium, the reaction temperature is 40-70°C, and the pressure in the autoclave is 8-12MPa , The drying time is 10-20 hours. 10. The method according to claim 1, characterized in that the silica-cellulose composite airgel prepared in step (5) has an apparent density of 0.092-0.143 g/cm ³ and a specific surface area of 227.41-264.93 m ² /g, and the thermal conductivity coefficient is between 0.029 and 0.038W/m·K.