A process for the preparation of ultra-high purity hydroxyl-free silicon dioxide

Abstract

The application relates to the technical field of nanomaterials, in particular to a process for preparing ultra-high-purity hydroxyl-free silicon dioxide, which comprises the following steps: S1, mixing a silicon source, an alkali and an organic modifier, then adding the mixture into a reaction medium to form a reaction solution, and performing microencapsulation treatment on the reaction solution to obtain reaction solution microcapsules; S2, adding the reaction solution microcapsules in a reaction process for synthesizing a composite frame material with a hollow structure, 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 pretreated precursor material; and S4, performing high-temperature treatment on the pretreated precursor material at 800-850 DEG C under the action of argon oscillation, then performing magnetic selection and screening, and thus high-purity hydroxyl-free silicon dioxide can be obtained. The synthesized silicon dioxide has small particle size, a round and regular structure and good dispersity, and can be applied to the fields of electronic packaging materials, mechanical lubrication, ceramics and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanomaterials, and particularly to a process for preparing ultra-high-purity hydroxyl-free silicon dioxide. BACKGROUND

[0002] Nanosilica has a wide range of applications in many fields, including filling, increment, thickening and reinforcing various organic materials, such as plastics, resins, rubbers, oils, etc. In order to solve the problem of powder flying 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 its surface make it hydrophilic, which makes it difficult to wet and disperse in organic phase, and the surface energy is large, and the aggregates tend to agglomerate, which affects the application performance of the product.

[0003] For example, Chinese patent CN104150490B discloses a preparation method of nanosilica, comprising: (1) adding triethanolamine and triethyl phosphate to water glass to obtain a mixed solution; (2) under stirring, adding sulfuric acid to the mixed solution of step (1) until the pH of the mixed solution is 5-6, and reacting for 20-30 minutes; (3) filtering the reaction solution of step (2), washing the filter residue, and drying at 110-120℃ to obtain nanosilica. This technical method can inhibit the agglomeration of particles, obtain silica with a particle size of 40-70nm, and also achieve hydroxyl-free by high-temperature treatment in the later stage. However, since the silica crystals can grow indefinitely in all directions during synthesis, the growth of the silica crystals is not limited, resulting in silica with different shapes and extremely irregular structures, and the impurities generated after the reaction are not easy to remove, resulting in low purity of the silica, which is not suitable for industrial needs. SUMMARY

[0004] In view of the problems in the prior art, the present application aims to provide a process for preparing ultra-high-purity hydroxyl-free silicon dioxide.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0006] A process for preparing ultra-high-purity hydroxyl-free silicon dioxide, specifically comprising the following steps:

[0007] S1 mixing a silicon source, an alkali and an organic modifier, and then adding them to a reaction medium to form a reaction solution, and performing microencapsulation treatment on the reaction solution to obtain a reaction solution microcapsule;

[0008] S2 adding the reaction solution microcapsule in the reaction process of synthesizing a composite framework material with a hollow structure, and performing high-temperature and high-pressure reaction to obtain a precursor material;

[0009] S3, by a hydrothermal method, deposits magnetic ferroferric oxide nanoparticles on the surface of the precursor material to obtain a pretreated precursor material;

[0010] S4, under oscillation, high-temperature treatment of the pretreated precursor material at 800-850 DEG C in argon, and then screening by magnetic separation, high-purity hydroxyl-free silicon dioxide is obtained.

[0011] As a further preferred embodiment of the present application, in S1, the silicon source is at least one of sodium silicate, sodium metasilicate, and quartz sand;

[0012] The base is at least one of sodium hydroxide, sodium carbonate, and sodium bicarbonate;

[0013] The organic modifier is any one of alkoxysilane, chlorosilane, nitrogen silane, and fluorosilane;

[0014] The reaction medium is water and / or an organic solvent, and the organic solvent is at least one of acetone and dimethylbenzene;

[0015] The silicon source, base, organic modifier, and reaction medium are in a molar ratio of 1: (0.08-0.16): (0.1-0.8): (15-23).

[0016] As a further preferred embodiment of the present application, in S1, the reaction liquid microcapsule is obtained by microencapsulation embedding treatment of the reaction liquid with whey protein and trehalose as wall materials.

[0017] Further, the reaction liquid microcapsule of S1 has the following specific operation method:

[0018] 1) Dissolve whey protein in distilled water, fully stir to obtain a whey protein solution, and then dissolve trehalose in distilled water to obtain a trehalose solution;

[0019] 2) Fully mix the whey protein solution, trehalose solution, and reaction liquid, and then perform spray drying under the following conditions: air flow rate 370-400 L / h, feed flow rate 0.4-0.6 L / h, inlet air temperature 120-123 DEG C, outlet air temperature 65-67 DEG C, and carrier gas pressure 0.1-0.2 MPa, to obtain the reaction liquid microcapsule;

[0020] The concentration of the whey protein solution is 90-150 g / L;

[0021] The concentration of the trehalose solution is 30-60 g / L;

[0022] The volume ratio of the whey protein solution, trehalose solution, and reaction liquid is 1:1: (0.3-0.6).

[0023] As a further preferred scheme of the present application, the specific operation of S2 is as follows:

[0024] 1) taking cobalt nitrate and 2-methylimidazole as raw materials, methanol as solvent, dissolving the two respectively, mixing them together, precipitating at room temperature for 24-30h to obtain a framework material;

[0025] 2) taking zinc nitrate as zinc source, sulfur powder as sulfur source, oleylamine and octylamine as solvent, fully mixing them, then adding the framework material, in a sealed hydrothermal reaction kettle filled with nitrogen, reacting at 160-165℃ for 5-7h to obtain a pretreated framework material;

[0026] 3) dispersing the pretreated framework material into ethanol, adding a reaction liquid microcapsule, fully stirring, then dispersing nickel nitrate into ethanol, fully stirring, mixing the two solutions, stirring for 30-50min, centrifuging with ethanol, drying the obtained product to obtain a composite framework material with core-shell structure;

[0027] 4) dispersing the composite framework material in methanol, adding ferrous chloride, stirring at room temperature for 90-120min, centrifuging with methanol, drying, standby, mixing the standby product, 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene and deionized water, ultrasonic treatment for 30-40min, then dropping 1-methyl-2-pyrrolidone, oscillating for 5-10min, and ultrasonic treatment for 5-10min, heating at 85-90℃ for 24-28h, washing and drying to obtain a cage structure material with hollow structure, then oscillating at 500-800r / min under 30-32MPa at 280-300℃ for 30-35h to obtain a precursor material.

[0028] Further, the specific operation of step 1) is as follows:

[0029] Dissolve cobalt nitrate and 2-methylimidazole in methanol respectively to obtain solution A and solution B, fully stir, quickly pour solution B into solution A, continuously stir at 100-150r / min for 30-50min, then precipitate at room temperature for 24-30h, centrifuge to collect the product, repeatedly wash with ethanol, and dry to obtain a framework material;

[0030] The ratio of cobalt nitrate, 2-methylimidazole and methanol is (5.8-7.5)g:(6.5-9.3)g:(100-150)mL.

[0031] Further, the specific operation of step 2) is as follows:

[0032] The zinc nitrate, sulfur powder, oleylamine and octylamine are mixed thoroughly, then the framework material is added, and after mixing, it is placed in a hydrothermal reaction kettle, nitrogen is filled for 5-10 min, then the hydrothermal reaction kettle is quickly packaged, and it is placed in a 160-165℃ oven for reaction for 5-7 h, 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 after drying, the pretreated framework material is obtained;

[0033] The ratio of the 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.

[0034] Further, in step 3), the ratio of the pretreated framework material, ethanol and reaction liquid microcapsules is (8-12) g:(4000-6000) mL:(2-5) g.

[0035] The ratio of the nickel nitrate and ethanol is (1.8-2.5) g:(100-150) mL.

[0036] The mass ratio of the pretreated framework material and nickel nitrate is (8-12):(1.8-2.5).

[0037] Further, in step 4), the ratio of the composite framework material, methanol, ferrous chloride, standby product, 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene, 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.

[0038] The stirring speed is 100-150 r / min.

[0039] The oscillation speed is 500-800 r / min.

[0040] The ultrasonic treatment power is 150-200 W.

[0041] As a further preferred scheme of the present application, the specific operation of S3 is as follows:

[0042] The ferrous sulfate and ferric citrate are dissolved in deionized water, the precursor material is added, after fully stirring and mixing, ascorbic acid is added, the pH value is adjusted to 9.0-9.5, after stirring, it is transferred to a hydrothermal reaction kettle, and reacted at 200-210℃ for 5-6 h, then the obtained product is centrifuged, and after magnetic separation, washing and drying, it is obtained.

[0043] 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.

[0044] As a further preferred scheme of the present application, the specific operation of S4 is as follows:

[0045] The pre-processed precursor material is put into a porcelain boat, under the action of oscillation, in an argon atmosphere, and heated to 800-830 DEG C and maintained for 2-3 h, after the processing is completed, the temperature is lowered to room temperature, then the magnetic separation is screened and centrifugal washing is performed, and after drying, the ultra-high purity hydroxyl-free silicon dioxide is obtained;

[0046] The oscillation is at a speed of 800-1200 r / min;

[0047] The heating is at a rate of 0.5-1.0 DEG C / min;

[0048] The cooling is at a rate of 5-8 DEG C / min.

[0049] Compared with the prior art, the present application has the beneficial effects of:

[0050] In the present application, the reaction solution is microencapsulated and embedded by using whey protein and trehalose as wall materials, and the reaction solution microcapsule is obtained; then, the framework material is synthesized by using cobalt nitrate and 2-methyl imidazole as raw materials, and zinc nitrate and sulfur powder are used as zinc source and sulfur source respectively, and the framework material is used as the base, and a large number of nanosheets with ultra-thin structure are deposited on the framework material through hydrothermal reaction, so that the pretreated framework material is obtained; then, the pretreated framework material and the reaction solution microcapsule are fully stirred, nickel ions are introduced, and a composite framework material with a core-shell structure is formed through reaction, and after the 2, 3, 6, 7, 10, 11-hexahydroxy triphenyl benzene ligand is introduced and the composite framework material is treated by solvent, a cage structure material with a hollow structure is obtained, and a large number of short nanorods branch out from the cage structure material, and the short nanorods have good supporting effect and can maintain the stability of the structure of the cage structure material to a great extent; the inside of the cage structure material is a hollow structure, and a large number of reaction solution microcapsules are reserved, and through mechanical stirring and ultrasonic action in the reaction, the reaction solution microcapsules move continuously in the hollow structure, and after contacting with the nanosheets deposited on the framework material, the nanosheets will break the wall material of the reaction solution microcapsule, so that the reaction solution is released, and then through oscillation reaction under high temperature and high pressure, the hydrolysis product of the silicon source in the reaction solution, metasilicic acid, is hydrolyzed to grow into a complete crystal lattice structure of silicon dioxide under the condition of high temperature and high pressure, and the organic modifier is combined on the surface of the nanocrystalline silicon dioxide to prevent the growth and aggregation of the nanoparticles, so as to control the size and dispersity, so that a precursor material containing silicon dioxide inside is obtained, and in the reaction process, the nanosheets deposited on the framework material can polish the formed silicon dioxide crystals, so that the corners of the silicon dioxide are polished, and the formed silicon dioxide structure is more round and regular; and through the reaction in the hollow structure of the cage structure material, the narrow space limits the growth and expansion range of the silicon dioxide crystals, and prevents the growth of the silicon dioxide crystals, so that the formation of silicon dioxide agglomerates can be reduced, and nanoscale silicon dioxide with small particles is formed; and in order to reduce the impurities in the silicon dioxide, in the present application, magnetic magnetite is deposited on the precursor material by hydrothermal method, and finally through high temperature treatment in an argon atmosphere, the length and diameter of the short nanorods in the cage structure material are reduced, the supporting effect of the short nanorods on the cage structure material is weakened, the structure of the cage structure material is broken and collapsed, so that the generated silicon dioxide inside can be released and treated, at the same time, the hydroxyl groups on the silicon dioxide are removed through dehydration reaction under high temperature in an argon atmosphere, so as to realize the hydroxyl-free of the silicon dioxide, and through subsequent magnetic separation and screening, the cage structure material can be cleaned out, so that high-purity hydroxyl-free silicon dioxide can be obtained.

[0051] In the present application, the reaction solution composed of a silicon source, an alkali, an organic modifier and a reaction medium is subjected to microencapsulation treatment to obtain reaction solution microcapsules, and the reaction solution microcapsules are added in the reaction process for synthesizing a composite framework material with a hollow structure, so that the growth of silica crystals is prevented by carrying out the reaction in the narrow space of the hollow structure, thereby reducing the formation of silica agglomerates, forming nanosilica with small particles, and through the deposition of nanosheets on the framework material, the silica crystals formed can be polished, the corners of the silica can be polished away, and the silica structure formed is more round and regular. DETAILED DESCRIPTION

[0052] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0053] In the embodiments of the present application, the silicon source is sodium silicate, the alkali is sodium hydroxide, the organic modifier is alkoxysilane, and the reaction medium is acetone.

[0054] Embodiment 1

[0055] A process for preparing ultra-high-purity hydroxyl-free silicon dioxide, specifically comprising the following steps:

[0056] S1-1) uniformly mix sodium silicate, sodium hydroxide, alkoxysilane and acetone in a molar ratio of 1:0.08:0.1:15 to form a reaction solution;

[0057] S1-2) dissolve whey protein in distilled water, obtain a whey protein solution with a concentration of 90 g / L after sufficient stirring, and then dissolve trehalose in distilled water to obtain a trehalose solution with a concentration of 30 g / L;

[0058] S1-3) mix the whey protein solution, the trehalose solution and the reaction solution in a volume ratio of 1:1:0.3, and then perform spray drying under the conditions of an air flow rate of 370 L / h, a feeding flow rate of 0.4 L / h, an air inlet temperature of 120℃, an air outlet temperature of 65℃ and a carrier gas pressure of 0.1 MPa to obtain reaction solution microcapsules;

[0059] S2-1) dissolve 5.8 g of cobalt nitrate and 6.5 g of 2-methylimidazole in 100 mL of methanol respectively to obtain A solution and B solution, pour the B solution into the A solution quickly after sufficient stirring, continuously stir at 100 r / min for 30 min, and then precipitate at room temperature for 24 h, centrifugally collect the product, and wash repeatedly with ethanol, and then obtain the framework material after drying;

[0060] S2-2) After mixing 1.0 mmol of zinc nitrate, 1.0 mmol of sulfur powder, 10 mL of oleylamine and 5 mL of octylamine, 1.0 g of the framework material was added, mixed uniformly and placed in a hydrothermal reactor, filled with nitrogen for 5 min, then the hydrothermal reactor was quickly sealed and placed in a 160°C oven for 5 h. After the reaction was completed, it was naturally cooled to room temperature, and the product was repeatedly washed with a mixture of methanol and chloroform, and dried to obtain a pretreated framework material;

[0061] S2-3) 8 g of the pretreated framework material was dispersed in 4000 mL of ethanol, then 2 g of the reaction liquid microcapsule was added, stirred thoroughly, 1.8 g of nickel nitrate was dispersed in 100 mL of ethanol and stirred thoroughly, the two solutions were mixed, stirred at 120 r / min for 30 min, and then centrifuged with ethanol. The obtained product was dried to obtain a composite framework material with a core-shell structure;

[0062] S2-4) 1.0 g of the composite framework material was dispersed in 30 mL of methanol, 2.0 g of ferrous chloride was added, and stirred at 100 r / min for 90 min at room temperature. After washing with methanol and drying, it was ready for use. 1.2 g of the ready-to-use product, 0.7 g of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene and 150 mL of deionized water were mixed, treated with 150 W ultrasonic for 30 min, then 16.8 mL of 1-methyl-2-pyrrolidone was added, oscillated at 500 r / min for 5 min, and treated with 150 W ultrasonic for 5 min. After heating at 85°C for 24 h, it was repeatedly washed with deionized water and acetone and dried to obtain a cage structure material with a hollow structure. Then, the precursor material was obtained by reacting at 280°C and 30 MPa for 30 h;

[0063] S3) 300 mmol of ferrous sulfate and 300 mmol of ferric citrate were dissolved in 100 mL of deionized water, 10 g of the precursor material was added, stirred thoroughly, 500 mmol of ascorbic acid was added, and 0.3 mol / L of sodium hydroxide solution was added to adjust the pH value to 9.0. After stirring at 400 r / min for 30 min, it was transferred to a hydrothermal reactor and reacted at 200°C for 5 h. The obtained product was centrifuged, magnetically separated, washed and dried to obtain a pretreated precursor material;

[0064] S4) The pretreated precursor material was placed in a porcelain boat, and under the action of 800 r / min oscillation, it was heated to 800°C at a rate of 0.5°C / min in an argon atmosphere, and maintained for 2 h. After the treatment was completed, it was cooled to room temperature at a rate of 5°C / min, and then screened and washed by magnetic separation and centrifugation. After drying, ultra-high purity hydroxyl-free silicon dioxide was obtained.

[0065] Example 2

[0066] A process for preparing ultra-high purity hydroxyl-free silicon dioxide, specifically comprising the following steps:

[0067] S1-1) uniformly mix sodium silicate, sodium hydroxide, alkoxysilane and acetone in a molar ratio of 1:0.12:0.5:18 to form a reaction solution;

[0068] S1-2) dissolve whey protein in distilled water, and after sufficient stirring, obtain a whey protein solution with a concentration of 120 g / L, then dissolve trehalose in distilled water to obtain a trehalose solution with a concentration of 50 g / L;

[0069] S1-3) mix the whey protein solution, the trehalose solution and the reaction solution in a volume ratio of 1:1:0.5, then perform spray drying 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 ℃, an outlet air temperature of 66 ℃ and a carrier gas pressure of 0.1 MPa to obtain reaction solution microcapsules;

[0070] S2-1) dissolve 6.7 g of cobalt nitrate and 8.0 g of 2-methylimidazole in 120 mL of methanol respectively to obtain solution A and solution B, after sufficient stirring, quickly pour solution B into solution A, continuously stir at 120 r / min for 40 min, then precipitate at room temperature for 28 h, centrifugally collect the product, repeatedly wash with ethanol, and after drying, obtain a framework material;

[0071] S2-2) mix 1.2 mmol of zinc nitrate, 1.2 mmol of sulfur powder, 13 mL of oleylamine and 7 mL of octylamine, then add 1.3 g of the framework material, mix uniformly, then place in a hydrothermal reaction kettle, fill with nitrogen for 7 min, then quickly seal the hydrothermal reaction kettle, and place it in a 162 ℃ oven for reaction for 6 h, after the reaction is completed, naturally cool to room temperature, repeatedly wash the product with a mixture of methanol and chloroform, and after drying, obtain a pretreated framework material;

[0072] S2-3) disperse 10 g of the pretreated framework material into 5000 mL of ethanol, then add 3 g of the reaction solution microcapsules, after sufficient stirring, disperse 2.1 g of nickel nitrate into 120 mL of ethanol, mix the two solutions after sufficient stirring, stir at 150 r / min for 40 min, centrifugally separate with ethanol, then after drying, obtain a composite framework material with a core-shell structure;

[0073] S2-4) 1.3 g of the composite framework material was dispersed in 40 mL of methanol, 2.5 g of ferrous chloride was added, and stirring was performed at 130 r / min for 100 min at room temperature. After centrifugal washing with methanol and drying, 1.5 g of the prepared product, 0.9 g of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene, and 170 mL of deionized water were mixed, and ultrasonic treatment was performed at 200 W for 35 min. Then, 17.2 mL of 1-methyl-2-pyrrolidone was added dropwise, and oscillation was performed at 700 r / min for 7 min, and ultrasonic treatment was performed at 200 W for 7 min. After heating at 87°C for 26 h, the hollow cage structure material was obtained after repeated washing with deionized water and acetone and drying. Then, the precursor material was obtained after reaction at 290°C and 31 MPa for 32 h;

[0074] S3) 350 mmol of ferrous sulfate and 350 mmol of ferric citrate were dissolved in 150 mL of deionized water, 12 g of the precursor material was added, and stirring was performed until uniform. Then, 580 mmol of ascorbic acid was added, and a 0.35 mol / L sodium hydroxide solution was added to adjust the pH value to 9.5. After stirring at 450 r / min for 40 min, the mixture was transferred to a hydrothermal reaction kettle, and reaction was performed at 205°C for 6 h. After centrifugal separation, magnetic separation, washing, and drying, the pretreated precursor material was obtained.

[0075] S4) The pretreated precursor material was placed in a porcelain boat, and oscillation was performed at 1000 r / min. After heating to 820°C at a rate of 1.0°C / min in an argon atmosphere and maintaining for 2.5 h, the temperature was decreased to room temperature at a rate of 7°C / min. After magnetic separation, centrifugal washing, and drying, the ultra-high-purity hydroxyl-free silicon dioxide was obtained.

[0076] Example 3

[0077] A process for preparing ultra-high-purity hydroxyl-free silicon dioxide, specifically comprising the following steps:

[0078] S1-1) Sodium silicate, sodium hydroxide, alkoxysilane, and acetone were uniformly mixed in a molar ratio of 1:0.16:0.8:23 to form a reaction solution.

[0079] S1-2) Whey protein was dissolved in distilled water to obtain a whey protein solution with a concentration of 150 g / L. Then, trehalose was dissolved in distilled water to obtain a trehalose solution with a concentration of 60 g / L.

[0080] S1-3) The whey protein solution, trehalose solution and reaction liquid were mixed in a volume ratio of 1:1:0.6, and then spray dried under the conditions of an air flow rate of 400 L / h, a feeding flow rate of 0.6 L / h, a feeding air temperature of 123°C, an outlet air temperature of 67°C, and a carrier gas pressure of 0.2 MPa to obtain reaction liquid microcapsules;

[0081] S2-1) 7.5 g of cobalt nitrate and 9.3 g of 2-methylimidazole were respectively dissolved in 150 mL of methanol to obtain solution A and solution B. After being stirred thoroughly, solution B was quickly poured into solution A. After being continuously stirred at 150 r / min for 50 min, the mixture was left to precipitate at room temperature for 30 h. The product was collected by centrifugation and repeatedly washed with ethanol. After being dried, the framework material was obtained.

[0082] S2-2) 1.5 mmol of zinc nitrate, 1.5 mmol of sulfur powder, 15 mL of oleylamine and 8 mL of octylamine were thoroughly mixed, and then 1.6 g of the framework material was added. After being mixed uniformly, the mixture was placed in a hydrothermal reaction kettle, and nitrogen was filled for 10 min. Then, the hydrothermal reaction kettle was quickly sealed and placed in an oven at 165°C for reaction for 7 h. After the reaction was completed, the product was naturally cooled to room temperature, repeatedly washed with a mixture of methanol and chloroform, and dried to obtain a pretreated framework material.

[0083] S2-3) 12 g of the pretreated framework material was dispersed in 6000 mL of ethanol, and then 5 g of the reaction liquid microcapsules was added. After being stirred thoroughly, 2.5 g of nickel nitrate was dispersed in 150 mL of ethanol and stirred thoroughly. The two solutions were mixed, stirred at 180 r / min for 50 min, and then separated by centrifugation with ethanol. After being dried, the obtained product was a composite framework material with a core-shell structure.

[0084] S2-4) 1.5 g of the composite framework material was dispersed in 50 mL of methanol, and then 2.8 g of ferrous chloride was added. After being stirred at 150 r / min for 120 min at room temperature, the product was washed by centrifugation with methanol and dried for standby use. 1.6 g of the standby product, 1.0 g of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene and 180 mL of deionized water were mixed. After being treated by ultrasonic waves at 200 W for 40 min, 17.5 mL of 1-methyl-2-pyrrolidone was added dropwise. After being oscillated at 800 r / min for 10 min and treated by ultrasonic waves at 200 W for 10 min, the mixture was heated at 90°C for 28 h. After being repeatedly washed with deionized water and acetone, the product was dried to obtain a cage-shaped material with a hollow structure. Then, the precursor material was obtained by reacting at 300°C and 32 MPa for 35 h.

[0085] S3) 400 mmol of ferrous sulfate and 400 mmol of ferric citrate were dissolved in 180 mL of deionized water, 13 g of the precursor material was added, after being fully stirred and mixed, 630 mmol of ascorbic acid was added, and 0.4 mol / L of sodium hydroxide solution was added to adjust the pH value to 9.5, after being stirred at 500 r / min for 50 min, it was transferred to a hydrothermal reaction kettle, reacted at 210°C for 6h, after the obtained product was centrifuged, separated by magnetic separation, washed and dried, the pretreated precursor material was obtained;

[0086] S4) The pretreated precursor material was placed in a porcelain boat, under the action of 1200 r / min oscillation, in an argon atmosphere, it was heated to 830°C at a rate of 1.0°C / min and maintained for 3h, after the treatment was completed, it was cooled to room temperature at a rate of 8°C / min, then it was screened and washed by magnetic separation and centrifugation, and after drying, ultra-high purity hydroxyl-free silicon dioxide was obtained.

[0087] Comparative Example 1: Sodium silicate, sodium hydroxide, alkoxysilane, and acetone were mixed uniformly in a molar ratio of 1:0.08:0.1:15 to form a reaction solution, which was reacted at 280°C and 30 MPa for 30h, and the product was washed and dried.

[0088] Comparative Example 2: This comparative example is basically the same as Example 1, except that step S2-1 is omitted.

[0089] Comparative Example 3: This comparative example is basically the same as Example 1, except that step S2-2 is omitted.

[0090] Comparative Example 4: This comparative example is basically the same as Example 1, except that step S3 is omitted.

[0091] Comparative Example 5: This comparative example is basically the same as Example 1, except that step S4 is omitted.

[0092] Test Experiment:

[0093] The average roundness of the silica particles was measured by the following method.

[0094] First, the primary particles of the silica particles dispersed in resin particles (polyester, weight average molecular weight Mw = 50,000) with a volume average particle size of 100 μm were observed by a SEM device, and the roundness of the silica particles was obtained as "100 / SF2" calculated from the following formula from the planar image analysis of the obtained primary particles.

[0095] Formula: Roundness (100 / SF2) = 4π×(A / I 2 )

[0096] [In formula (1), I represents the circumference length of the primary particles in the image, and A represents the projected area of the primary particles.]

[0097] In addition, the average roundness of the silica particles is obtained as the 50% roundness in the cumulative frequency of the roundness of 100 primary particles obtained by the above planar image analysis.

[0098] According to the method provided in Examples 1-3 and Comparative Examples 1-5, silica nanoparticles are respectively processed, then infrared spectrum test is performed on the silica nanoparticles to observe whether hydroxyl is contained, and according to the above method, the average roundness of the silica particles is calculated, and at the same time, the purity of the obtained silica nanoparticles is determined by ICP method, and the results are shown in Table 1.

[0099] Table 1

[0100]

[0101] Note: There is no result in Comparative Example 5 because the obtained product is not silica nanoparticles.

[0102] As can be seen from Table 1, the silica obtained in the present application has high purity, no hydroxyl, round and regular structure, good dispersibility, and has good market application prospect.

[0103] The above disclosed preferred embodiments of the present application are only used to help explain the present application. The preferred embodiments do not describe all the details, nor limit the present application to the specific embodiments described. Obviously, according to the content of the present application, many modifications and changes can be made. The present application selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well understand and utilize the present application. The present application is limited only by the claims and their full scope and equivalents.

Claims

1. A process for the preparation of ultra-high purity hydroxyl-free silicon dioxide, characterized in that, Specifically comprising the following steps: S1mixing the silicon source, alkali and organic modifier, then adding them into the reaction medium to form a reaction solution, and microencapsulating the reaction solution to obtain a reaction solution microcapsule; S2adding the reaction solution microcapsule into the reaction process of synthesizing the composite framework material with hollow structure, and obtaining a precursor material through high temperature and high pressure reaction; The specific operation is as follows: 1) taking cobalt nitrate and 2-methyl imidazole as raw materials, methanol as solvent, dissolving them respectively, then mixing them together, and precipitating at room temperature for 24-30 hours to obtain a framework material; 2) taking zinc nitrate as zinc source, sulfur powder as sulfur source, and oleylamine and octylamine as solvent, mixing them thoroughly, then adding the framework material, and reacting in a sealed hydrothermal reaction kettle filled with nitrogen at 160-165℃ for 5-7 hours to obtain a pretreated framework material; 3) dispersing the pretreated framework material into ethanol, adding the reaction solution microcapsule, stirring thoroughly, then dispersing nickel nitrate into ethanol and stirring thoroughly, mixing the two solutions, stirring for 30-50 minutes, centrifuging with ethanol, drying the obtained product to obtain a composite framework material with core-shell structure; 4) dispersing the composite framework material into methanol, adding ferrous chloride, stirring at room temperature for 90-120 minutes, washing with methanol, drying, standby, mixing the standby product, 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene and deionized water, ultrasonic treating for 30-40 minutes, dropping into 1-methyl-2-pyrrolidone, oscillating for 5-10 minutes, ultrasonic treating for 5-10 minutes, heating at 85-90℃ for 24-28 hours, washing, drying, obtaining a cage structure material with hollow structure, then oscillating at 500-800r / min at 280-300℃ and 30-32MPa for 30-35 hours to obtain a precursor material; S3depositing magnetic ferroferric oxide nanoparticles on the surface of the precursor material through a hydrothermal method to obtain a pretreated precursor material; S4high-temperature treating the pretreated precursor material at 800-850℃ under the action of oscillation in argon, then screening through magnetic selection to obtain high-purity hydroxyl-free silicon dioxide.

2. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized by, 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 organic solvent, and the organic solvent is at least one of acetone and xylene; The molar ratio of the silicon source, alkali, organic modifier and reaction medium is 1: (0.08-0.16): (0.1-0.8): (15-23).

3. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, characterized by, In S1, the reaction solution microcapsule is obtained by microencapsulating the reaction solution with whey protein and trehalose as wall materials.

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

5. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, wherein 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.

6. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, wherein In step 3), the ratio of pretreated framework material, ethanol, and reaction liquid microcapsules is (8-12) g:(4000-6000) mL:(2-5) g. The ratio of nickel nitrate and ethanol is (1.8-2.5) g:(100-150) mL. The mass ratio of pretreated framework material and nickel nitrate is (8-12):(1.8-2.5).

7. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, wherein In step 4), the ratio of composite framework material, methanol, ferrous chloride, standby product, 2,3,6,7,10,11-hexahydroxytriphenylbenzene, 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 ultrasonic treatment power is 150-200 W.

8. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, wherein S3, the specific operation is as follows: Ferrous sulfate and ferric citrate are dissolved in deionized water, and precursor material is added. After fully stirring and mixing, ascorbic acid is added, and the pH value is adjusted to 9.0-9.

5. After stirring, it is transferred to a hydrothermal reaction kettle and reacted at 200-210℃ for 5-6h. The obtained product is centrifuged, magnetically separated, washed and dried. 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.

9. The process for preparing ultra-high purity hydroxyl-free silicon dioxide according to claim 1, wherein S4, the specific operation is as follows: The pretreated precursor material is placed in a porcelain boat, and under the action of oscillation, it is heated to 800-830℃ in an argon atmosphere and maintained for 2-3h. After the treatment is completed, it is cooled to room temperature, and then it is magnetically selected, screened, and centrifugally washed. After drying, ultra-high-purity hydroxyl-free silicon dioxide is obtained. The oscillation speed is 800-1200 r / min. The heating rate is 0.5-1.0℃ / min. The cooling rate is 5-8℃ / min.

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