CN1626440A - Method for preparing Nano silicon dioxide - Google Patents
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Abstract
A process for preparing nano-SiO2 includes such steps as adding water-soluble polymer and/or water-soluble organic C1-C8 alcohol to the solution alkali-metal silicate, stirring adding weak organic acid as catalyst, hydrolyzing reaction to obtain the hydrosol, washing, filter, and natural drying or supercritical extracting. The obtained amorphous nano-SiO2 has high purity and dispersity and large specific surface area.
Description
Technical field
The present invention relates to a kind of preparation method of nano silicon dioxide, particularly prepare the method for nano silicon from alkali-metal silicate.
Background technology
The normal synthetic method that adopts of the preparation of nano silicon has the precipitator method and colloidal sol～gel method (Sol-Gel method).Colloidal sol～gel method is a raw material with the organic alcohol ester of silicic acid, costs an arm and a leg, and its production cost is very high, difficult realization large-scale industrial production.Synthesize with the precipitator method, can adopt cheap alkalimetal silicate (water glass) is raw material, but generally all adopt strongly-acid mineral acid and alkalimetal silicate to carry out precipitin reaction, as: Chinese patent 94117058 discloses the method for a kind of " precipitated silica ", with alkalimetal silicate and mineral acid precipitin reaction, after filtration, precipitation, washing, drying obtain the silica dioxide granule of particle size range between the 150-250 nanometer.This method is with acid stronger mineral acid hydrolysis precipitation, and the generation and the speed of growth thereof of silicic acid sol glue nuclear are wayward, and particle is easily reunited during washing, is difficult to obtain the nanometer grade silica particle.
Chinese patent 97125800 discloses a kind of " method for preparing nanometer silicon dioxide particle from alkali-metal silicate ", with surfactant dissolves in non-polar organic solvent, wiring solution-forming, prepare the aqueous solution of alkalimetal silicate according to the requirement of the size of nano SiO 2 particle, this aqueous solution stirring is joined in the non-polar organic solvent solution of tensio-active agent down, make the inverse micellar solution of alkalimetal silicate; This inverse micellar solution is added in the acidated polar organic solvent disperse phase, and nanometer silicon dioxide particle is made in ageing, separation.This patent is used surfactant package polysilicate particle in reaction, form microemulsified, and the growth of control particulate obtains the nanometer grade silica particle.But be to use the exhibiting high surface promoting agent to be difficult to wash off, need complicated aftertreatment technology, otherwise influence the purity of product, and handle, cost height, unsuitable large-scale industrial production with the tensio-active agent microemulsified.
Summary of the invention
When using alkalimetal silicate and Acid precipitation prepared in reaction nano silicon in order to solve, agglomeration traits easily takes place in the wayward and aftertreatment of particle diameter, the present invention proposes a kind of method for preparing nano silica, this method can form molecular network and isolate silicic acid sol glue nuclear in alkali metal silicate solutions precipitin reaction system, thereby control and the nucleus growth of prevention glue, and the aftertreatment technology to colloidal sol is simple, make the fluffy generation of nano particle be difficult for reuniting suitability for industrialized production.
Main points of the present invention are: utilize water-soluble polymers or water-soluble organic alcohol to form the complicated high-molecule chelated network system in alkali metal silicate solutions, with the slightly acidic organic acid is the hydrolytic precipitation agent, make to be dispersed in high-molecule chelated network system mesosilicic acid root and to precipitate lentamente and separate out, thus the growth of control silicic acid sol glue nuclear.Main technical schemes: in alkali metal silicate solutions, add water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol, stirring in the silicate solutions forms the high-molecule chelated network system; Under 40-98 ℃, with C
1-C
8The organic carboxyl acid reactant aqueous solution, the organic carboxyl acid concentration of aqueous solution is: carboxylic acid accounts for carboxylic acid aqueous solution weight percentage 20-80wt%, reaches 3.5-5.5 until PH and obtains silica gel colloidal sol; With the silicic acid sol that is obtained deionized water wash, filter, then with absolute ethanol washing for several times, contained moisture in displacing colloidal sol after filtration, has obtained silicic acid/ethanol gel; Drying obtains nano silicon xerogel powder again;
Wherein water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol is shared volumetric concentration 0.1g/L at least in silicate solutions.
The water-soluble polymers that aforesaid method uses is polyoxyethylene glycol (PEG) or the polypropylene glycol (PPG) of weight-average molecular weight between 200-19000, or the polyvinyl alcohol of weight-average molecular weight between 300-150000, or the polyoxyethylene of weight-average molecular weight between 300-200000, or carboxymethyl cellulose, Natvosol (HEC) and the hydroxypropylcellulose (HPC) of weight-average molecular weight between 200-25000, or the polyvinylpyrrolidone (PVP) of weight-average molecular weight between 1000-1000000.
The water-soluble C that aforesaid method uses
1-C
8Organic alcohol is to be selected from C
1-C
8Aliphatic monohydric alcohol, C
2-C
8Aliphatic diol, C
3-C
8Aliphatic tribasic alcohol, C
3-C
8Aliphatic series tetravalent alcohol, C
3-C
8Aliphatics alcohols in the aliphatic series pentavalent alcohol or their mixture;
The organic carboxyl acid that aforesaid method uses is a formic acid, acetate, propionic acid, butyric acid, valeric acid, caproic acid, toxilic acid, citric acid.
After obtaining silicic acid ethanol gel in the aforesaid method, employing supercritical extraction drying, cook extractant with low-carbon alcohol, silicic acid ethanol gel joined be made into titanium dioxide silanol glue gel in the low-carbon alcohol, in moving in the autoclave, at 260 ℃-300 ℃, under the super critical condition of 7.0-9.5MPa, after stablizing 0.5-1.0h, release fluids gets nano silica powder;
After obtaining silicic acid ethanol gel in the aforesaid method, adopt the supercritical extraction drying, at supercritical CO
2Extracting in the medium, liquid carbon dioxide enters in the reactor and circulates, at 30 ℃-70 ℃, pressure 5MPa-35Mpa, CO
2Rate of flow of fluid 5kg/h-20kg/h, supercritical CO
2Fluid 10-200h cycling time stops supercritical CO
2After the fluid circulation, emptying CO
2Gas gets nano silica powder.
Effect of the present invention: present method adds high molecular weight water soluble polymer in alkalimetal silicate and Acid precipitation reaction system and/or organic alcohol forms the macromolecular network system, the silicic acid sol glue nuclear phase that hydrolytic precipitation generates is isolated mutually, thereby play control and stop glue to examine the effect of further growth; With organic acid is the hydrolytic precipitation catalyzer and since acidity a little less than, reduced the caryogenic speed of silicic acid sol glue, reaction generates the water-sol of nanometer particle; Be replaced as ethanol colloidal sol through absolute ethanol washing, make nano particle fluffy, effectively prevent particle agglomeration; Select seasoning or enforceable supercritical fluid drying, had≤mean particle size of 50nm 200-1000m
2The specific surface area of/g, purity＞99.5%, the amorphous nano silicon of fluffy, polymolecularity, and product structure is easy to control, the aperture evenly and narrow distribution range, controllable specific surface area.Preparation process is simple, and is with low cost, applied widely.
Embodiment:
The preparation of silica gel colloidal sol:
A) alkali metal silicate solutions of present method employing is for containing Na
2O and SiO
2Water glass solution, Na wherein
2The weight percentage 1.0-10wt% of O in solution, SiO
2Weight percentage 1.0-30wt% in solution adds water-soluble polymers and/or water-soluble C in above-mentioned water glass solution
1-C
8Organic alcohol, water-soluble polymers shared volumetric concentration in silicate solutions is 0.1g/L-10g/L, water-soluble C
1-C
8Organic alcohol shared volumetric concentration in silicate solutions is 0.1g/L-10g/L, stirs, and is warmed up to the temperature between 40-98 ℃, drips C
1-C
8The organic carboxyl acid aqueous solution, carboxylic acid account for carboxylic acid aqueous solution weight percentage 20-80wt%, till pH reaches 3.5-5.5, continue to stir 0.1-8.0 hour acquisition silica gel colloidal sol; Or:
Containing the Na of 1.0-10wt%
2O and 1.0-30wt%SiO
2Water glass solution in, add water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol, water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol shared volumetric concentration in silicate solutions plays the effect of regulating control precipitin reaction glue nuclear size, water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol shared volumetric concentration in silicate solutions is 0.1g/L-10g/L, under the whipped state, above-mentioned mixing solutions is added drop-wise to the C of the 20-80wt% concentration of temperature between 40-98 ℃
1-C
18The organic carboxyl acid aqueous solution till pH reaches 3.5-5.5, continues to stir 0.1-8.0 hour acquisition silica gel colloidal sol;
In this step, add water-soluble polymers, in water glass solution, form the high-molecule chelated network system, the colloidal sol glue nuclear that precipitin reaction is become plays iris action, selected water-soluble polymers with soluble in water and solubleness is bigger, and removed easily again through washing and roasting and to be principle.Preferred water-soluble polymers is polyoxyethylene glycol (PEG) or the polypropylene glycol (PPG) of weight-average molecular weight between 200-19000, or the polyvinyl alcohol of weight-average molecular weight between 300-150000, or the polyoxyethylene of weight-average molecular weight between 300-200000, carboxymethyl cellulose, Natvosol (HEC) and the hydroxypropylcellulose (HPC) of weight-average molecular weight between 200-25000, and the polyvinylpyrrolidone (PVP) of weight-average molecular weight between 1000-1000000;
It is identical with above-mentioned adding water-soluble polymers role to add organic alcohol in this step, the water-soluble C that selects for use
1-C
8Organic alcohol is to be selected from C
1-C
8Aliphatic monohydric alcohol, C
2-C
8Aliphatic diol, C
3-C
8Aliphatic tribasic alcohol, C
3-C
8Aliphatic series tetravalent alcohol, C
3-C
8Aliphatics alcohols in the aliphatic series pentavalent alcohol or their mixture, as: ethylene glycol, glycerol, neopentyl glycol, TriMethylolPropane(TMP), tetramethylolmethane, ring pentanediol, Xylitol, sorbyl alcohol, glycol ether, dipropylene glycol or their mixture.Water-soluble polymers and organic alcohol also can use together.
In order to control the growth of silicic acid sol glue nuclear, present method is used the weak acid hydrolysis, makes to be dispersed in high-molecule chelated network system mesosilicic acid root and to precipitate lentamente and separate out, and the organic carboxyl acid of selecting for use is: formic acid, acetate, propionic acid, butyric acid, valeric acid, caproic acid, toxilic acid, citric acid.
Aftertreatment to silicic acid sol:
B) with the silicic acid sol that is obtained deionized water wash, filter, with absolute ethanol washing for several times, fully replace moisture contained in the colloidal sol then, alcohol is washed and can be prevented the reunion of nano particle when aftertreatment, filters once more after alcohol is washed, and has obtained silicic acid/ethanol gel;
C) with step B) in the gel that obtains air-dry, obtained the silica xerogel powder of nano particle size.In addition, also can adopt methods such as hig h-speed centrifugal spray drying, vacuum-drying, expansion drying to carry out drying.
D) with step C) product be silicon-dioxide powdery in 300 ℃ of-500 ℃ of temperature ranges roasting 0.5-10 hour, obtain the unbodied nano silica powder of stabilization, have≤mean particle size of 50nm 200-1000m
2The specific surface area of/g.
C in the above-mentioned embodiment) step can be used the supercritical extraction drying mode, with narrow, the uniform nano silica powder of the particle distribution range that obtains bigger serface, duct prosperity.Two kinds of embodiments are arranged:
1, use the low-carbon alcohol extracting: at step B) in the gel that obtains carry out silicic acid ethanol gel behind the solvent exchange several with an amount of dehydrated alcohol, join and be made into titanium dioxide silanol glue gel in the dehydrated alcohol, in moving in the autoclave, at 260 ℃-300 ℃, 7.0-9.5MPa super critical condition under, make extraction solvent with dehydrated alcohol, stablize 0.5-1.0h after, release fluids gets narrow, the uniform nano silica powder of particle distribution range of bigger serface, duct prosperity.
Except that dehydrated alcohol, suitable extraction solvent can also be low-carbon alcohol such as methyl alcohol, Virahol with solvent in extracting in the supercritical extraction process.Its consumption is not strict with, and generally determines according to the building-up reactions economy, extraction solvent content can be controlled in the scope of 20%-80% of whole extracting mixture weight, and optimum control is at 30%-50%.
2, supercritical co extracting: CO
2Through the liquid CO of refrigeration compressor boil down to
2, temperature is subzero 5 ℃, pressure 4-5Mpa in the vaporizer, squeezes into high-temperature preheater through high-pressure metering pump, temperature is 30-70 ℃, liquid CO
2After being preheated, entering in the autoclave that silicic acid/ethanol gel filter cake is housed and circulate CO
2Flow velocity is 5kg/h-20kg/h, and the autoclave temperature is that 30 ℃-70 ℃, pressure are 5MPa-35Mpa, CO
2Supercutical fluid carries ethanol by the autoclave extraction, enters separator and carries out CO
2/ ethanol separates, and separator temperature is 0-5 ℃, pressure 4-5Mpa, and the ethanol after the separation is by the valve emptying of separator below, CO
2Gas enters omnibus bar by the valve of separator top, enters condensate compressor once more and circulates.When the temperature and pressure at each position reaches controlling valu, the beginning computation cycles time,, period 10-200 hour, finish until the ethanol extracting according to the ethanol content of silicic acid/ethanol gel.Stop CO
2The terminal valve of autoclave is closed in circulation, opens blow-off valve, treat that pressure reduces after, drive still and take out silica xerogel, place 500 ℃ of roasting 0.5-5h of High Temperature Furnaces Heating Apparatus, nano silica powder.
Embodiment 1:
Take by weighing Na
2O and SiO
2Percentage composition is respectively 8.2% and 26% water glass solution 107.33g, joins in the deionized water of 568ml, is made into Na
2O concentration is rare water glass solution of 0.25mol/l, takes by weighing polyoxyethylene glycol-10000 5.2g and joins in the above-mentioned water glass solution, stirs dissolving fully; Press Na
2O/HAc=1.0: 6.0 (mol ratios) take by weighing 51.16g acetic acid and add water to be mixed with the 1.0mol/L acetum stand-by; Constantly stir down, with the mixing solutions heat temperature raising to 80 of water glass and polyoxyethylene glycol-10000 ℃, be added dropwise to the 1.0mol/L acetum for preparing, 2.5h dropwise, continue to stir white transparent suspension liquid, ageing 1 hour stops to heat nature and reduces to room temperature, and filtration, deionized water washing are to the neutral and no Ac of detection
-1Behind the ion, fully replace three times, behind filtration, the natural drying at room temperature,, get white nano silica powder in 500 ℃ of roasting 2h with dehydrated alcohol.Median size is 35.2nm, and the specific surface area that BET analyzes is 298.6m
2/ g.
Embodiment 2:
Take by weighing Na
2O and SiO
2Percentage composition is respectively 7.06% and 22.58% concentrated sodium silicate solution 100kg, adds deionized water 455.45kg and is diluted to Na
2O concentration is rare water glass solution of 0.25mol/L, takes by weighing polyoxyethylene glycol-2000 4.5kg and joins in the above-mentioned water glass solution, stirs dissolving fully; Press Na
2O/HAc=1.0: 5.0 (mol ratios) take by weighing 98% acetic acid 34.86kg and add deionized water 142.34kg and be made into the 4.0mol/L acetum; Constantly stir down, with 4.0mol/L acetum heat temperature raising to 75 ℃, be added dropwise to the mixing solutions of water glass and polyoxyethylene glycol-2000,2.5h dropwise, continue to stir white transparent suspension liquid, ageing 1 hour stops to heat nature and reduces to room temperature, the pH value is 4.59, and filtration, deionized water washing are to the neutral and no Ac of detection
-1Behind the ion, fully replace three times, filter with dehydrated alcohol, add an amount of absolute ethyl alcohol and stirring pulp suspension liquid, leave standstill 12h after, squeezing into the hot air flow temperature through Mono pump is in 100 ℃ the Highspeedcentrifugingandsprayingdrier after the drying, in 500 ℃ of roasting 2h, get white nano silica powder.Median size is 25.5nm, and the specific surface area that BET analyzes is 796.9m
2/ g.
Embodiment 3:
Take by weighing Na
2O and SiO
2Percentage composition is respectively 6.0% and 19.16% water glass solution 85.0kg, joins in the deionized water of 387kg, is made into Na
2O concentration is rare water glass solution of 0.25mol/L, takes by weighing polyoxyethylene glycol-10000 5.2kg and joins in the above-mentioned water glass solution, stirs dissolving fully; Press Na
2O/HAc=1.0: 6.0 (mol ratios) take by weighing 98% acetic acid 23.7kg and add deionized water 387kg and be made into the 1.0mol/L acetum; Constantly stir down, with the mixing solutions heat temperature raising to 40 of water glass and polyoxyethylene glycol-10000 ℃, be added dropwise to the 1.0mol/L acetum for preparing, 2.5h dropwise, continue to stir white transparent suspension liquid, ageing 1 hour stops to heat nature and reduces to room temperature, the pH value is 4.8, and Plate Filtration, deionized water washing are to the neutral and no Ac of detection
-1Behind the ion, fully replace three times, filter, add an amount of absolute ethyl alcohol and stirring pulp suspension liquid with dehydrated alcohol, after leaving standstill 12h, squeezing into the hot air flow temperature through Mono pump is in 100 ℃ the Highspeedcentrifugingandsprayingdrier after the drying, in 500 ℃ of roasting 2h, white nano silica powder.Median size is 35.5nm, and the specific surface area that BET analyzes is 198.9m
2/ g.
Embodiment 4:
Adopt the step identical with embodiment 1, the water-soluble polymers that adds in the water glass solution is Polyvinylpyrolidone (PVP) (PVP-K30) 5.2g, presses Na
2O/HAc=1.0: 4.0 (mol ratios), take by weighing 34.16g acetic acid and add water and be mixed with the 1.0mol/L acetum, be prepared into white nano silica powder.Median size is 25.2nm, and the specific surface area that BET analyzes is 226.7m
2/ g.
Embodiment 5:
Adopt the step identical,, be prepared into white nano silica powder interpolation ethylene glycol 15.2g in the water glass solution with embodiment 1.Median size is 38.2nm, and the specific surface area that BET analyzes is 268.7m
2/ g.
Embodiment 6:
Adopt the step identical,, be prepared into white nano silica powder adding polyvinyl alcohol-7500 5g and ethylene glycol 20g in the water glass solution with embodiment 1.Median size is 26.4nm, and the specific surface area that BET analyzes is 245.7m
2/ g.
Embodiment 7:
Adopt the step identical, change acetum into 1.0mol/L formic acid solution 65.2g with embodiment 1, must white nano silica powder.Median size is 25.7nm, and the specific surface area that BET analyzes is 316.8m
2/ g.
Embodiment 8:
Adopt the step identical,, be prepared into white nano silica powder interpolation TriMethylolPropane(TMP) 15.2g in the water glass solution with embodiment 1.Median size is 28.2nm, and the specific surface area that BET analyzes is 288.7m
2/ g.
Embodiment 9:
Adopt the step identical,, be added dropwise to the 1.0mol/l acetum for preparing, be prepared into white nano silica powder the mixing solutions heat temperature raising to 40 of water glass and polyoxyethylene glycol-10000 ℃ with embodiment 1.Median size is 28.8nm, and the specific surface area that BET analyzes is 268.7m
2/ g.
Embodiment 10:
Take by weighing Na
2O and SiO
2Percentage composition is respectively 8.2% and 26% water glass solution 107.33g, joins in the deionized water of 568ml, is made into Na
2O concentration is rare water glass solution of 0.25mol/L, takes by weighing polyoxyethylene glycol-10000 5.2g and joins in the above-mentioned water glass solution, stirs dissolving fully; Press Na
2O/HAc=1.0: 6.0 (mol ratios), 51.16g is stand-by for preparation 1.0mol/L acetum; Constantly stir down, with the mixing solutions heat temperature raising to 80 of water glass and polyoxyethylene glycol-10000 ℃, be added dropwise to the 1.0mol/l acetum for preparing, 2.5h dropwise, continue to stir white transparent suspension liquid, ageing 1 hour stops to heat nature and reduces to room temperature, and filtration, deionized water washing are to the neutral and no Ac of detection
-1Behind the ion, obtain silicic acid sol, fully replace three times with dehydrated alcohol, filter, add an amount of absolute ethyl alcohol and stirring pulp suspension liquid, move in the autoclave in, at 260 ℃-300 ℃, 7.0-9.5MPa super critical condition under, make extraction solvent with dehydrated alcohol, stablize 0.5-1.0h after, release fluids gets nano silica powder.Median size is 18.2nm, and the specific surface area that BET analyzes is 346.7m
2/ g.
Embodiment 11:
Adopt the step identical with embodiment 2, hydrosol of silicic acid is fully replaced with dehydrated alcohol and is converted into silicic acid/ethanol colloidal sol for three times, and filtration moves into filter cake in the autoclave, is that supercritical medium carries out the supercritical extraction drying with the carbonic acid gas.At first, CO
2Through the liquid CO of refrigeration compressor boil down to
2, temperature is subzero 5 ℃, pressure 5Mpa in the vaporizer, squeezes into high-temperature preheater through high-pressure metering pump, temperature is 40 ℃, liquid CO
2After being preheated, entering in the autoclave that silicic acid/ethanol gel filter cake is housed and circulate CO
2Flow velocity is 10/h, and the autoclave temperature is that 40 ℃, pressure are 10Mpa, CO
2Supercutical fluid carries ethanol by the autoclave extraction, enters separator and carries out CO
2/ ethanol separates, and separator temperature is 1 ℃, pressure 5Mpa, and the ethanol after the separation is by the valve emptying of separator below, CO
2Gas enters omnibus bar by the valve of separator top, enters condensate compressor once more and circulates.Circulated 20 hours, and finished until the ethanol extracting.Stop CO
2The terminal valve of autoclave is closed in circulation, opens blow-off valve, treat that pressure reduces after, drive still and take out silica xerogel, place 500 ℃ of roasting 2h of High Temperature Furnaces Heating Apparatus, nano silica powder.Median size is 14.7nm, and the specific surface area that BET analyzes is 857.6m
2/ g.
Claims (8)
1, a kind of preparation method of nano silicon dioxide, carry out precipitin reaction with alkali metal silicate solutions and acid and prepare silicic acid sol, after filtration, washing, dry preparation nano-powder, it is characterized in that: in alkali metal silicate solutions, add water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol, stirring forms the high-molecule chelated network system; Under 40-98 ℃, with C
1-C
8The organic carboxyl acid reactant aqueous solution obtains silicic acid sol, and the organic carboxyl acid concentration of aqueous solution is: carboxylic acid accounts for carboxylic acid aqueous solution weight percentage 20-80wt%; With the silicic acid sol that is obtained deionized water wash, filter, use absolute ethanol washing then, contained moisture in displacing colloidal sol after filtration, has obtained silicic acid/ethanol gel; Drying obtains nano silicon xerogel powder again; Wherein water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol shared volumetric concentration in silicate solutions is at least 0.1g/L.
2, preparation method according to claim 1 is characterized in that: water-soluble polymers and/or water-soluble C
1-C
8Organic alcohol shared volumetric concentration in silicate solutions is 0.1g/L-10g/L.
3, preparation method according to claim 1, it is characterized in that: water-soluble polymers is polyoxyethylene glycol or the polypropylene glycol of weight-average molecular weight between 200-19000, or the polyvinyl alcohol of weight-average molecular weight between 300-150000, or the polyoxyethylene of weight-average molecular weight between 300-200000, or carboxymethyl cellulose, Natvosol and the hydroxypropylcellulose of weight-average molecular weight between 200-25000, or the Polyvinylpyrolidone (PVP) of weight-average molecular weight between 1000-1000000.
4, preparation method according to claim 1 is characterized in that: the water-soluble C of use
1-C
8Organic alcohol is to be selected from C
1-C
8Aliphatic monohydric alcohol, C
2-C
8Aliphatic diol, C
3-C
8Aliphatic tribasic alcohol, C
3-C
8Aliphatic series tetravalent alcohol, C
3-C
8Aliphatics alcohols in the aliphatic series pentavalent alcohol or their mixture;
5, preparation method according to claim 1 is characterized in that: the organic carboxyl acid of use is a formic acid, acetate, propionic acid, butyric acid, valeric acid, caproic acid, toxilic acid, citric acid.
6, preparation method according to claim 1, it is characterized in that: to the silicic acid/ethanol gel that obtains, adopt supercritical extraction technology drying: silicic acid ethanol gel is joined be made into the silicic acid alcosol in the low-carbon alcohol, move in the autoclave, make extraction solvent with low-carbon alcohol, extraction solvent is controlled in the scope of 20%-80% of whole extracting mixture weight, at 260 ℃-300 ℃, 7.0-9.5MPa super critical condition under, after stablizing 0.5-1.0h, release fluids gets nano silica powder.
7, preparation method according to claim 6 is characterized in that: the low-carbon alcohol extraction solvent is selected dehydrated alcohol, methyl alcohol, Virahol for use.
8, preparation method according to claim 1 is characterized in that: after obtaining silicic acid ethanol gel, adopt supercritical CO
2Medium extracting drying, liquid carbon dioxide enters in the reactor and circulates, CO
2Flow velocity is 5kg/h-20kg/h, and temperature of reaction kettle is that 30 ℃-70 ℃, pressure are 5MPa-35Mpa, supercritical CO
2Fluid 10-200h cycling time, after stopping to circulate, emptying CO
2Gas gets nano silica powder.
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