Process for preparing ultrahigh-purity hydroxyl-free silicon dioxide

Abstract

The invention relates to the technical field of nano materials, in particular to a process for preparing ultra-high-purity hydroxyl-free silicon dioxide, which comprises the following steps: S1, mixing a silicon source, alkali and an organic modifier, adding the mixture into a reaction medium to form a reaction liquid, and carrying out microencapsulation treatment on the reaction liquid to obtain reaction liquid microcapsules; s2, in the reaction process of synthesizing the composite frame material with the hollow structure, adding the reaction liquid microcapsules, and performing high-temperature and high-pressure reaction to obtain a precursor material; s3, depositing magnetic ferroferric oxide nanoparticles on the surface of the precursor material through a hydrothermal method to obtain a pretreated precursor material; and S4, in argon, under the action of oscillation, performing high-temperature treatment on the pretreated precursor material at 800-850 DEG C, and then performing magnetic separation screening to obtain the high-purity hydroxyl-free silicon dioxide. The synthesized silicon dioxide is small in particle size, round and regular in structure and good in dispersity, and can be applied to the industries of electronic packaging materials, mechanical lubrication, ceramics and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and particularly to a process for preparing ultra-high purity hydroxyl-free silica. Background Art

[0002] Nanosilica has a wide range of uses in many fields, including filling, incrementing, thickening, and reinforcing various organic materials, such as plastics, resins, rubbers, oils, etc. To solve the problems of powder flying and difficulty in dispersion during the processing of nanosilica, due to the presence of hydroxyl groups on its surface, the polysiloxane inside the nanosilica and the active silanol groups and adsorbed water on the outer surface make it hydrophilic, difficult to wet and disperse in the organic phase, with a relatively high surface energy, and the aggregates always tend to agglomerate, thus affecting the application performance of the product.

[0003] For example, Chinese Patent CN104150490B discloses a preparation method of nanosilica, including: (1) adding triethanolamine and triethyl phosphate to water glass to obtain a mixed solution; (2) adding sulfuric acid to the mixed solution in step (1) under stirring until the pH of the mixed solution is 5 - 6, and reacting for 20 - 30 minutes; (3) filtering the reaction solution in step (2), washing the filter residue, and drying at 110 - 120°C to obtain nanosilica. Although this technical method can inhibit the agglomeration of particles and obtain silica with a particle size of 40 - 70 nm, and can also achieve hydroxyl-free through subsequent high-temperature treatment, during the synthesis process, the silica crystal can grow infinitely in all directions, resulting in unrestricted growth of the silica crystal, causing the formed silica to have various shapes, with prominent edges and corners, and an extremely irregular structure. Moreover, the impurities generated after the reaction are not easily removed, resulting in a relatively low purity of the silica, which is not suitable for industrial requirements. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a process for preparing ultra-high purity hydroxyl-free silica.

[0005] To achieve the above purpose, the present invention provides the following technical solutions: A process for preparing ultra-high purity hydroxyl-free silica specifically includes the following steps: S1 Mix a silicon source, an alkali, and an organic modifier, then add them to a reaction medium to form a reaction solution, and perform microencapsulation treatment on the reaction solution to obtain a reaction solution microcapsule; S2 During the reaction process of synthesizing a composite framework material with a hollow structure, add the reaction solution microcapsule, and obtain a precursor material through high-temperature and high-pressure reaction; S3 Deposit magnetic iron tetroxide nanoparticles on the surface of the precursor material by hydrothermal method to obtain a pretreated precursor material; S4 In argon, under the action of oscillation, the pre-treated precursor material is subjected to high temperature treatment at 800-850°C, and then sieving through magnetic separation to obtain high-purity hydroxyl-free silicon dioxide.

[0006] As a further preferred embodiment of the present invention, in S1, the silicon source is at least one of sodium silicate, sodium metasilicate, and quartz sand; The alkali is at least one of sodium hydroxide, sodium carbonate and sodium bicarbonate; The organic modifier is any one of alkoxysilane, chlorosilane, nitrogen silane and fluorosilane; The reaction medium is water and / or an organic solvent, and the organic solvent is at least one of acetone and xylene; The molar ratio of the silicon source, the base, the organic modifier and the reaction medium is 1:(0.08-0.16):(0.1-0.8):(15-23).

[0007] As a further preferred embodiment of the present invention, in S1, the reaction solution microcapsules use whey protein and trehalose as wall materials, and the reaction solution is subjected to microencapsulation and embedding treatment to obtain reaction solution microcapsules.

[0008] Furthermore, the specific operation method of the reaction liquid microcapsule of S1 is as follows: 1) Dissolving whey protein in distilled water and stirring thoroughly to obtain a whey protein solution, and then dissolving trehalose in distilled water to obtain a trehalose solution; 2) The whey protein solution, the trehalose solution and the reaction solution are fully mixed, and then spray-dried under the conditions of an air flow rate of 370-400 L / h, a feed flow rate of 0.4-0.6 L / h, an inlet air temperature of 120-123°C, an outlet air temperature of 65-67°C, and a carrier gas pressure of 0.1-0.2 MPa to obtain reaction solution microcapsules; The whey protein solution has a concentration of 90-150 g / L; The trehalose solution has a concentration of 30-60 g / L; The volume ratio of the whey protein solution, trehalose solution and reaction solution is 1:1:(0.3-0.6).

[0009] As a further preferred embodiment of the present invention, the specific operation of S2 is as follows: 1) Using cobalt nitrate and 2-methylimidazole as raw materials and methanol as solvent, the two are dissolved separately, mixed together, and precipitated at room temperature for 24-30 hours to obtain a framework material; 2) Using zinc nitrate as a zinc source, sulfur powder as a sulfur source, oleylamine and octylamine as solvents, fully mix and add the framework material, react at 160-165° C. for 5-7 hours in a sealed hydrothermal reactor filled with nitrogen to obtain a pretreated framework material; 3) Disperse the pretreated framework material in ethanol, add the reaction liquid microcapsules, and stir well. Then disperse nickel nitrate in ethanol and stir well. Mix the two solutions and stir for 30 - 50 min. After centrifuging and separating with ethanol, dry the obtained product to obtain the core - shell structured composite framework material; 4) Disperse the composite framework material in methanol, add ferrous chloride, and stir at room temperature for 90 - 120 min. After centrifuging and washing with methanol and drying, set aside. Mix the reserved product, 2,3,6,7,10,11 - hexahydroxytriphenylene benzene, and deionized water. After ultrasonic treatment for 30 - 40 min, drop in 1 - methyl - 2 - pyrrolidone, oscillate for 5 - 10 min, and then ultrasonic treat for 5 - 10 min. Heat at 85 - 90 °C for 24 - 28 h. After washing and drying, obtain the cage - type structure material with a hollow structure. Then, at 280 - 300 °C and 30 - 32 MPa, oscillate and react at 500 - 800 r / min for 30 - 35 h to obtain the precursor material.

[0010] Furthermore, the specific operation of step 1) is as follows: Dissolve cobalt nitrate and 2 - methylimidazole in methanol respectively to obtain solution A and solution B. After stirring well, quickly pour solution B into solution A, continuously stir at 100 - 150 r / min for 30 - 50 min, then precipitate at room temperature for 24 - 30 h, centrifuge to collect the product, and wash it repeatedly with ethanol. After drying, obtain the framework material; The ratio of cobalt nitrate, 2 - methylimidazole, and methanol is (5.8 - 7.5) g:(6.5 - 9.3) g:(100 - 150) mL.

[0011] Furthermore, the specific operation of step 2) is as follows: Fully mix zinc nitrate, sulfur powder, oleylamine, and octylamine, add the framework material, mix well and place it in a hydrothermal reaction kettle. After filling with nitrogen for 5 - 10 min, quickly seal the hydrothermal reaction kettle and put it into an oven at 160 - 165 °C for reaction for 5 - 7 h. After the reaction ends, naturally cool to room temperature. Wash the product repeatedly with a mixed solution of methanol and chloroform, and after drying, obtain the pretreated framework material; The ratio of zinc nitrate, sulfur powder, oleylamine, octylamine, and the framework material is (1.0 - 1.5) mmol:(1.0 - 1.5) mmol:(10 - 15) mL:(5 - 8) mL:(1.0 - 1.6) g.

[0012] Furthermore, in step 3), the ratio of the pretreated framework material, ethanol, and the reaction liquid microcapsules is (8 - 12) g:(4000 - 6000) mL:(2 - 5) g; The ratio of nickel nitrate to ethanol is (1.8 - 2.5) g : (100 - 150) mL; The mass ratio of the pretreated framework material to nickel nitrate is (8 - 12) : (1.8 - 2.5).

[0013] Furthermore, in step 4), the ratio of the composite framework material, methanol, ferrous chloride, the spare product, 2,3,6,7,10,11 - hexahydroxytriphenylene benzene, deionized water, and 1 - methyl - 2 - pyrrolidone is (1.0 - 1.5) g : (30 - 50) mL : (2.0 - 2.8) g : (1.2 - 1.6) g : (0.7 - 1.0) g : (150 - 180) mL : (16.8 - 17.5) mL; The stirring speed is 100 - 150 r / min; The oscillation speed is 500 - 800 r / min; The power of the ultrasonic treatment is 150 - 200 W.

[0014] As a further preferred embodiment of the present invention, the specific operation of S3 is as follows: Dissolve ferrous sulfate and ferric citrate in deionized water, add the precursor material, stir well and then add ascorbic acid, adjust the pH value to 9.0 - 9.5, transfer to a hydrothermal reaction kettle after stirring, react at 200 - 210 °C for 5 - 6 h, centrifuge and separate the obtained product, and then perform magnetic separation, washing, and drying; The ratio of ferrous sulfate, ferric citrate, deionized water, the precursor material, and ascorbic acid is (300 - 400) mmol : (300 - 400) mmol : (100 - 180) mL : (10 - 13) g : (500 - 630) mmol.

[0015] As a further preferred embodiment of the present invention, the specific operation of S4 is as follows: Put the pretreated precursor material into a porcelain boat, under the action of oscillation, in an argon atmosphere, heat up to 800 - 830 °C and maintain for 2 - 3 h. After the treatment is completed, cool down to room temperature, and then obtain ultra - high - purity hydroxyl - free silica through magnetic separation screening, centrifugal washing, and drying; The oscillation speed is 800 - 1200 r / min; The heating rate is 0.5 - 1.0 °C / min; The cooling rate is 5 - 8 °C / min.

[0016] Compared with the prior art, the beneficial effects of the present invention are: In the present invention, whey protein and trehalose are used as wall materials, and a reaction solution composed of a silicon source, an alkali, an organic modifier, and a reaction medium is subjected to microencapsulation to obtain reaction solution microcapsules; then, cobalt nitrate and 2-methylimidazole are used as raw materials to synthesize a framework material, and zinc nitrate and sulfur powder are used as a zinc source and a sulfur source respectively. Using the framework material as a substrate, a large number of ultrathin nanosheets are deposited on the framework material through a hydrothermal reaction to obtain a pretreated framework material; then, the pretreated framework material and the reaction solution microcapsules are fully stirred, and nickel ions are introduced to form a composite framework material with a core-shell structure through reaction. After introducing 2,3,6,7,10,11-hexahydroxytriphenylene benzene ligand and performing solvothermal treatment with the composite framework material, a cage-like structure material with a hollow structure is obtained, and a large number of short nanorods branch out from the cage-like structure material. The formed short nanorods have a good supporting effect and can largely maintain the stability of the structure of the cage-like structure material; the inside of the cage-like structure material is a hollow structure, and a large number of reaction solution microcapsules are retained. By applying mechanical stirring and ultrasonic action during the reaction, the reaction solution microcapsules continuously move in the hollow structure. After contacting the nanosheets deposited on the framework material, the nanosheets will cut through the wall material of the reaction solution microcapsules, causing the reaction solution to be released. Then, through an oscillating reaction under high temperature and high pressure, metasilicic acid, the hydrolysis product of the silicon source in the reaction solution, hydrolyzes and grows into silica with a complete lattice structure under high temperature and high pressure conditions. At the same time, the organic modifier binds to the surface of the crystalline nano-silica, preventing the growth and aggregation of nanoparticles, thereby controlling the size and dispersibility, and thus obtaining a precursor material containing silica inside. And during the reaction with oscillation, the nanosheets deposited on the framework material can polish the formed silica crystals, and can polish off the corners of the silica, making the formed silica structure more round and regular; and by reacting in the hollow structure of the cage-like structure material, the narrow space limits the growth and expansion range of the silica crystals, preventing the growth of the silica crystals, thereby reducing the formation of silica agglomeration and making small particle nano-silica formed; and, in order to reduce the impurities in the silica, in the present invention, magnetic iron tetroxide is deposited on the precursor material by a hydrothermal method, and finally through high-temperature treatment in an argon atmosphere, the length and diameter of the short nanorods in the cage-like structure material are reduced, resulting in a weakened supporting effect on the cage-like structure material, causing the structure of the cage-like structure material to break and collapse, thereby releasing the internally generated silica for treatment. At the same time, under high-temperature treatment in an argon atmosphere, the hydroxyl groups on the silica undergo a dehydration reaction and are removed, thereby achieving the dehydroxylation of silica. And through subsequent magnetic separation and screening, the cage-like structure material can be removed, and thus high-purity dehydroxylated silica can be obtained.

[0017] In the present invention, a reaction liquid composed of a silicon source, an alkali, an organic modifier and a reaction medium is subjected to microencapsulation treatment to obtain reaction liquid microcapsules, and the reaction liquid microcapsules are added during the reaction process of synthesizing a composite framework material with a hollow structure. By reacting in a narrow space of the hollow structure, the growth of silicon dioxide crystals is prevented, thereby reducing the formation of silicon dioxide agglomerates, so that small particles of nano silicon dioxide are formed, and by depositing nanosheets on the framework material, the formed silicon dioxide crystals can be polished, and the edges and corners of the silicon dioxide can be polished off, so that the formed silicon dioxide structure is more rounded and regular. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] In the embodiment of the present invention, the silicon source is sodium silicate; the alkali is sodium hydroxide; the organic modifier is alkoxysilane; and the reaction medium is acetone.

[0020] Example 1 A process for preparing ultra-high purity hydroxyl-free silicon dioxide comprises the following steps: S1-1) mixing sodium silicate, sodium hydroxide, alkoxysilane and acetone in a molar ratio of 1:0.08:0.1:15 to form a reaction solution; S1-2) dissolving whey protein in distilled water, stirring thoroughly to obtain a whey protein solution with a concentration of 90 g / L, and then dissolving trehalose in distilled water to obtain a trehalose solution with a concentration of 30 g / L; S1-3) fully mixing the whey protein solution, the trehalose solution and the reaction solution in a volume ratio of 1:1:0.3, and then spray drying under the conditions of an air flow rate of 370 L / h, a feed flow rate of 0.4 L / h, an inlet air temperature of 120°C, an outlet air temperature of 65°C, and a carrier gas pressure of 0.1 MPa to obtain reaction solution microcapsules; S2-1) Dissolve 5.8 g of cobalt nitrate and 6.5 g of 2-methylimidazole in 100 mL of methanol to obtain solution A and solution B, respectively. After sufficient stirring, quickly pour solution B into solution A, continue stirring at 100 r / min for 30 min, precipitate at room temperature for 24 h, collect the product by centrifugation, wash it repeatedly with ethanol, and dry it to obtain the framework material; S2-2) After thoroughly mixing 1.0 mmol of zinc nitrate, 1.0 mmol of sulfur powder, 10 mL of oleylamine, and 5 mL of octylamine, add 1.0 g of the framework material. After mixing evenly, place it in a hydrothermal reaction kettle. After filling with nitrogen for 5 min, quickly seal the hydrothermal reaction kettle and put it into an oven at 160 °C for reaction for 5 h. After the reaction is completed, naturally cool it to room temperature. Wash the product repeatedly with a mixed solution of methanol and chloroform, and dry it to obtain the pretreated framework material; S2-3) Disperse 8 g of the pretreated framework material into 4000 mL of ethanol, then add 2 g of the reaction liquid microcapsule. After stirring thoroughly, disperse 1.8 g of nickel nitrate into 100 mL of ethanol and stir well. Mix the two solutions, stir at 120 r / min for 30 min, separate by centrifugation with ethanol, and dry the obtained product to obtain the core-shell structured composite framework material; S2-4) Disperse 1.0 g of the composite framework material in 30 mL of methanol, add 2.0 g of ferrous chloride, stir at 100 r / min at room temperature for 90 min, wash by centrifugation with methanol and then dry for standby. Mix 1.2 g of the standby product, 0.7 g of 2,3,6,7,10,11-hexahydroxytriphenylene benzene, and 150 mL of deionized water. After ultrasonic treatment at 150 W for 30 min, drop in 16.8 mL of 1-methyl-2-pyrrolidone, oscillate at 500 r / min for 5 min, and ultrasonic treat at 150 W for 5 min. Then heat at 85 °C for 24 h. After washing repeatedly with deionized water and acetone and drying, obtain the cage-structured material with a hollow structure. Then react at 280 °C and 30 MPa for 30 h to obtain the precursor material; S3) Dissolve 300 mmol of ferrous sulfate and 300 mmol of ferric citrate in 100 mL of deionized water, add 10 g of the precursor material, stir and mix evenly, add 500 mmol of ascorbic acid, and add 0.3 mol / L sodium hydroxide solution to adjust the pH value to 9.0. After stirring at 400 r / min for 30 min, transfer it to a hydrothermal reaction kettle and react at 200 °C for 5 h. After centrifuging the obtained product, perform magnetic separation, washing, and drying to obtain the pretreated precursor material; S4) Place the pretreated precursor material in a porcelain boat. Under the action of oscillation at 800 r / min, heat it to 800 °C at a rate of 0.5 °C / min in an argon atmosphere and maintain for 2 h. After the treatment is completed, cool it to room temperature at a rate of 5 °C / min. Then, through magnetic separation screening, centrifugal washing, and drying, ultra-high purity hydroxyl-free silica can be obtained.

[0021] Example 2 A process for preparing ultra-high purity hydroxyl-free silica specifically includes the following steps: S1-1) mixing sodium silicate, sodium hydroxide, alkoxysilane and acetone in a molar ratio of 1:0.12:0.5:18 to form a reaction solution; S1-2) dissolving whey protein in distilled water, stirring thoroughly to obtain a whey protein solution with a concentration of 120 g / L, and then dissolving trehalose in distilled water to obtain a trehalose solution with a concentration of 50 g / L; S1-3) the whey protein solution, the trehalose solution and the reaction solution are fully mixed at a volume ratio of 1:1:0.5, and then spray-dried under the conditions of an air flow rate of 380 L / h, a feed flow rate of 0.5 L / h, an inlet air temperature of 121° C., an outlet air temperature of 66° C., and a carrier gas pressure of 0.1 MPa to obtain reaction solution microcapsules; S2-1) 6.7 g of cobalt nitrate and 8.0 g of 2-methylimidazole were dissolved in 120 mL of methanol to obtain solution A and solution B, respectively. After sufficient stirring, solution B was quickly poured into solution A, and the mixture was stirred at 120 r / min for 40 min. The mixture was precipitated at room temperature for 28 h, and the product was collected by centrifugation, and repeatedly washed with ethanol. The framework material was obtained after drying. S2-2) After 1.2mmol zinc nitrate, 1.2mmol sulfur powder, 13mL oleylamine and 7mL octylamine are fully mixed, 1.3g framework material is added, the mixture is placed in a hydrothermal reactor, nitrogen is filled for 7 minutes, the hydrothermal reactor is quickly sealed, and it is placed in a 162°C oven for reaction for 6 hours. After the reaction is completed, it is naturally cooled to room temperature, the product is repeatedly washed with a mixture of methanol and chloroform, and dried to obtain a pretreated framework material; S2-3) Disperse 10g of the pretreated framework material into 5000mL of ethanol, then add 3g of the reaction solution microcapsule, stir thoroughly, then disperse 2.1g of nickel nitrate into 120mL of ethanol and stir thoroughly, mix the two solutions, stir at 150r / min for 40min, centrifuge with ethanol, and dry the obtained product to obtain a composite framework material with a core-shell structure; S2-4) Disperse 1.3g of the composite framework material in 40mL of methanol, add 2.5g of ferrous chloride, stir at 130r / min for 100min at room temperature, wash with methanol by centrifugation and dry for use, mix 1.5g of the standby product, 0.9g of 2,3,6,7,10,11-hexahydroxytriphenylene benzene with 170mL of deionized water, treat with 200W ultrasound for 35min, drop 17.2mL of 1-methyl-2-pyrrolidone, oscillate at 700r / min for 7min, and treat with 200W ultrasound for 7min, heat at 87°C for 26h, wash repeatedly with deionized water and acetone and dry to obtain a cage-type structure material with a hollow structure, and then react at 290°C and 31MPa for 32h to obtain a precursor material; S3) 350mmol of ferrous sulfate and 350mmol of ferric citrate were dissolved in 150mL of deionized water, 12g of the precursor material was added, and after being fully stirred and mixed, 580mmol of ascorbic acid was added, and 0.35mol / L of sodium hydroxide solution was added to adjust the pH value to 9.5. After stirring at 450r / min for 40min, the mixture was transferred to a hydrothermal reactor and reacted at 205°C for 6h. The obtained product was centrifuged, magnetically separated, washed and dried to obtain a pretreated precursor material; S4) The pre-treated precursor material is placed in a porcelain boat, and under the action of 1000r / min oscillation, the temperature is increased to 820°C at a rate of 1.0°C / min in an argon atmosphere, and maintained for 2.5 hours. After the treatment is completed, the temperature is lowered to room temperature at a rate of 7°C / min, and then through magnetic separation and centrifugal washing, ultra-high purity hydroxyl-free silica can be obtained after drying.

[0022] Example 3 A process for preparing ultra-high purity hydroxyl-free silicon dioxide comprises the following steps: S1-1) mixing sodium silicate, sodium hydroxide, alkoxysilane and acetone in a molar ratio of 1:0.16:0.8:23 to form a reaction solution; S1-2) dissolving whey protein in distilled water, stirring thoroughly to obtain a whey protein solution with a concentration of 150 g / L, and then dissolving trehalose in distilled water to obtain a trehalose solution with a concentration of 60 g / L; S1-3) fully mixing the whey protein solution, the trehalose solution and the reaction solution at a volume ratio of 1:1:0.6, and then spray drying under the conditions of an air flow rate of 400 L / h, a feed flow rate of 0.6 L / h, an inlet air temperature of 123° C., an outlet air temperature of 67° C., and a carrier gas pressure of 0.2 MPa to obtain reaction solution microcapsules; S2-1) Dissolve 7.5 g of cobalt nitrate and 9.3 g of 2-methylimidazole in 150 mL of methanol to obtain solution A and solution B, respectively. After sufficient stirring, quickly pour solution B into solution A, continue stirring at 150 r / min for 50 min, precipitate at room temperature for 30 h, collect the product by centrifugation, wash it repeatedly with ethanol, and dry it to obtain the framework material; S2-2) After 1.5mmol zinc nitrate, 1.5mmol sulfur powder, 15mL oleylamine and 8mL octylamine are fully mixed, 1.6g framework material is added, the mixture is placed in a hydrothermal reactor, nitrogen is filled for 10 minutes, the hydrothermal reactor is quickly sealed, and it is placed in a 165°C oven for reaction for 7 hours. After the reaction is completed, it is naturally cooled to room temperature, the product is repeatedly washed with a mixture of methanol and chloroform, and dried to obtain a pretreated framework material; S2-3) Disperse 12 g of the pretreated framework material into 6000 mL of ethanol, then add 5 g of the reaction solution microcapsules. After stirring well, disperse 2.5 g of nickel nitrate into 150 mL of ethanol and stir thoroughly. Mix the two solutions, stir at 180 r / min for 50 min, centrifuge and separate with ethanol, and then dry the obtained product to obtain the core-shell structured composite framework material; S2-4) Disperse 1.5 g of the composite framework material in 50 mL of methanol, add 2.8 g of ferrous chloride, stir at 150 r / min at room temperature for 120 min, centrifuge and wash with methanol and then dry for standby. Mix 1.6 g of the standby product, 1.0 g of 2,3,6,7,10,11-hexahydroxytriphenylene benzene and 180 mL of deionized water. After ultrasonic treatment at 200 W for 40 min, drop in 17.5 mL of 1-methyl-2-pyrrolidone, oscillate at 800 r / min for 10 min, and after ultrasonic treatment at 200 W for 10 min, heat at 90 °C for 28 h. After washing repeatedly with deionized water and acetone and then drying, obtain the cage-structured material with a hollow structure, and then react at 300 °C and 32 MPa for 35 h to obtain the precursor material; S3) Dissolve 400 mmol of ferrous sulfate and 400 mmol of iron citrate in 180 mL of deionized water, add 13 g of the precursor material, stir well and mix evenly, add 630 mmol of ascorbic acid, and add 0.4 mol / L sodium hydroxide solution to adjust the pH value to 9.5. After stirring at 500 r / min for 50 min, transfer to a hydrothermal reaction kettle and react at 210 °C for 6 h. After centrifuging and separating the obtained product, perform magnetic separation, washing and drying to obtain the pretreated precursor material; S4) Place the pretreated precursor material in a porcelain boat, under the action of oscillation at 1200 r / min, in an argon atmosphere, heat up to 830 °C at a rate of 1.0 °C / min and maintain for 3 h. After the treatment is completed, cool down to room temperature at a rate of 8 °C / min, and then through magnetic separation screening and centrifugal washing, dry it to obtain ultra-high purity silica without hydroxyl groups.

[0023] Comparative Example 1: Mix sodium silicate, sodium hydroxide, alkoxysilane, and acetone evenly according to a molar ratio of 1:0.08:0.1:15 to form a reaction solution, react at 280 °C and 30 MPa for 30 h, and wash and dry the product.

[0024] Comparative Example 2: This comparative example is basically the same as Example 1, except that step S2-1 is omitted.

[0025] Comparative Example 3: This comparative example is basically the same as Example 1, except that step S2-2 is omitted.

[0026] Comparative Example 4: This comparative example is basically the same as Example 1, except that step S3 is omitted.

[0027] Comparative Example 5: This comparative example is basically the same as Example 1, except that step S4 is omitted.

[0028] Test experiment: The average roundness of the silica particles was measured by the following method.

[0029] First, the primary particles after dispersing the silica particles into resin particles (polyester, weight-average molecular weight Mw = 50,000) with a volume-average particle size of 100 μm were observed by an SEM device. From the analysis of the planar image of the obtained primary particles, the roundness of the silica particles was obtained as "100 / SF2" calculated by the following formula.

[0030] Formula: Roundness (100 / SF2) = 4π×(A / I 2 ) [In formula (1), I represents the circumferential length of the primary particles in the image, and A represents the projected area of the primary particles.] In addition, the average roundness of the silica particles was obtained as the 50% roundness in the cumulative frequency of the roundness of 100 primary particles obtained through the above planar image analysis.

[0031] According to the methods provided in Examples 1-3 and Comparative Examples 1-5, silica nanoparticles were processed respectively. Then, infrared spectroscopy tests were carried out on the silica nanoparticles to observe whether hydroxyl groups were contained. And according to the above method, the average roundness of the silica particles was calculated. At the same time, the purity of the obtained silica nanoparticles was determined by ICP method. The results are shown in Table 1.

[0032] Table 1 Note: There is no result in Comparative Example 5 because the obtained product is not silica nanoparticles.

[0033] It can be seen from Table 1 that the silica obtained in the present invention has high purity, no hydroxyl groups, a round and regular structure, good dispersibility, and has good market application prospects.

[0034] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A process for preparing ultra-high purity hydroxyl-free silicon dioxide, characterized in that: The specific steps include: S1: mixing a silicon source, a base and an organic modifier and adding the mixture to a reaction medium to form a reaction solution, and performing microencapsulation treatment on the reaction solution to obtain reaction solution microcapsules; S2 adds reaction liquid microcapsules during the reaction process of synthesizing the composite framework material with a hollow structure, and obtains the precursor material through high temperature and high pressure reaction; S3 deposits magnetic ferroferric oxide nanoparticles on the surface of the precursor material by a hydrothermal method to obtain a pretreated precursor material; S4 In argon, under the action of oscillation, the pre-treated precursor material is subjected to high temperature treatment at 800-850°C, and then sieving through magnetic separation to obtain high-purity hydroxyl-free silicon dioxide.

2. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized in that: In S1, the silicon source is at least one of sodium silicate, sodium metasilicate, and quartz sand; The alkali is at least one of sodium hydroxide, sodium carbonate and sodium bicarbonate; The organic modifier is any one of alkoxysilane, chlorosilane, nitrogen silane and fluorosilane; The reaction medium is water and / or an organic solvent, and the organic solvent is at least one of acetone and xylene; The molar ratio of the silicon source, the base, the organic modifier and the reaction medium is 1:(0.08-0.16):(0.1-0.8):(15-23).

3. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized in that: In S1, the reaction solution microcapsules use whey protein and trehalose as wall materials, and the reaction solution is subjected to microencapsulation and embedding treatment to obtain reaction solution microcapsules.

4. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized in that: S2, the specific operations are as follows: 1) Using cobalt nitrate and 2-methylimidazole as raw materials and methanol as solvent, the two are dissolved separately, mixed together, and precipitated at room temperature for 24-30 hours to obtain a framework material; 2) Using zinc nitrate as a zinc source, sulfur powder as a sulfur source, oleylamine and octylamine as solvents, fully mix and add the framework material, react at 160-165° C. for 5-7 hours in a sealed hydrothermal reactor filled with nitrogen to obtain a pretreated framework material; 3) Disperse the pretreated framework material in ethanol, add the reaction solution microcapsules, stir thoroughly, disperse nickel nitrate in ethanol and stir thoroughly, mix the two solutions and stir for 30-50 minutes, centrifuge with ethanol, and dry the obtained product to obtain a composite framework material with a core-shell structure; 4) The composite framework material is dispersed in methanol, ferrous chloride is added, and the mixture is stirred at room temperature for 90-120 min. The mixture is centrifugally washed with methanol and then dried for later use. The reserved product, 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene benzene and deionized water are mixed, and ultrasonic treatment is performed for 30-40 min. 1-methyl-2-pyrrolidone is added dropwise, and the mixture is oscillated for 5-10 min and ultrasonic treatment is performed for 5-10 min. The mixture is heated at 85-90° C. for 24-28 h, and then washed and dried to obtain a cage-type structure material with a hollow structure. The mixture is then oscillated at 280-300° C. and 30-32 MPa at 500-800 r / min for 30-35 h to obtain a precursor material.

5. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 4, characterized in that: In step 1), the ratio of cobalt nitrate, 2-methylimidazole and methanol is (5.8-7.5) g: (6.5-9.3) g: (100-150) mL.

6. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 4, characterized in that: In step 2), the ratio of zinc nitrate, sulfur powder, oleylamine, octylamine and framework material is (1.0-1.5) mmol: (1.0-1.5) mmol: (10-15) mL: (5-8) mL: (1.0-1.6) g.

7. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 4, characterized in that: In step 3), the ratio of the pre-treated framework material, ethanol, and reaction solution microcapsules is (8-12) g: (4000-6000) mL: (2-5) g; The ratio of nickel nitrate to ethanol is (1.8-2.5) g: (100-150) mL; The mass ratio of the pre-treated frame material and nickel nitrate is (8-12): (1.8-2.5).

8. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 4, characterized in that: In step 4), the ratio of the composite framework material, methanol, ferrous chloride, spare product, 2, 3, 6, 7, 10, 11-hexahydroxytriphenylenebenzene, deionized water, and 1-methyl-2-pyrrolidone is (1.0-1.5) g: (30-50) mL: (2.0-2.8) g: (1.2-1.6) g: (0.7-1.0) g: (150-180) mL: (16.8-17.5) mL; The stirring speed is 100-150r / min; The oscillation speed is 500-800r / min; The power of the ultrasonic treatment is 150-200W.

9. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized in that: S3, the specific operations are as follows: Dissolve ferrous sulfate and ferric citrate in deionized water, add the precursor material, stir well and then add ascorbic acid, adjust the pH value to 9.0-9.5, transfer to a hydrothermal reactor after stirring, react at 200-210°C for 5-6h, centrifuge the obtained product, and then perform magnetic separation, washing and drying; The ratio of ferrous sulfate, ferric citrate, deionized water, precursor material and ascorbic acid is (300-400) mmol: (300-400) mmol: (100-180) mL: (10-13) g: (500-630) mmol.

10. A process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized in that: S4, the specific operations are as follows: The pre-treated precursor material is placed in a porcelain boat, and the temperature is raised to 800-830°C in an argon atmosphere under oscillation, and maintained for 2-3 hours. After the treatment is completed, the temperature is lowered to room temperature, and then the ultra-high purity hydroxyl-free silica is obtained after magnetic separation, screening, centrifugal washing, and drying. The oscillation speed is 800-1200r / min; The heating rate is 0.5-1.0°C / min; The cooling rate is 5-8°C / min.

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