Silicon dioxide microsphere as well as preparation method and application thereof

Abstract

The invention discloses a silicon dioxide microsphere as well as a preparation method and application thereof. The silicon dioxide microspheres are formed by self-assembly construction of silicon dioxide nanospheres with the diameter of 10-250 nm as basic units, the overall diameter of the microspheres is 0.5-15 microns, a large number of fine pore channels exist in the microspheres, and the microspheres have the advantages of being high in specific surface area (80-400 m / g), suitable in pore diameter (5-20 nm) and large in pore volume (0.3-1.2 cm < 3 > / g). The silicon dioxide microsphere prepared by the method has both a macroscopic microsphere form and a microcosmic porous structure, is low in metal impurity content, shows high column efficiency when being used as a chromatographic separation filler, has good catalytic performance when being used as a CO catalytic oxidation catalyst carrier, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of inorganic nanomaterials, and particularly relates to a porous silica microsphere, a preparation method thereof, and application of the porous silica microsphere in chromatographic separation filler and CO catalytic oxidation catalyst. BACKGROUND

[0002] As an important inorganic functional material, silica microspheres play an indispensable role in many fields such as chromatographic separation, catalysis, drug delivery, electronic information, and high-end coatings due to their excellent chemical stability, thermal stability, good mechanical strength, controllable pore size, and easily modified surface.

[0003] In the field of chromatographic separation, silica microspheres are the most core stationary phase filler in high-performance liquid chromatography (HPLC) and the most advanced ultra-high-performance liquid chromatography (UHPLC). The performance of silica microspheres directly determines the separation effect. An ideal chromatographic filler needs to meet several key requirements: ① regular spherical morphology to ensure uniform packing, stable column bed, and low column pressure; ② high specific surface area to provide sufficient sample loading capacity; ③ appropriate and uniform pore size to allow rapid diffusion of analyte molecules and interaction with the stationary phase; and ④ good mechanical strength to withstand the impact of high-pressure systems. Currently, commercial chromatographic fillers are mostly solid or amorphous silica microspheres prepared by the Stöber method or its improved methods. Although these microspheres have good sphericity and uniform size, their specific surface area is relatively limited (usually < 50 m² / g), and they lack internal through-order pores, resulting in large mass transfer resistance, which may lead to chromatographic peak broadening and difficulty in further improving separation efficiency (theoretical plate number). Although some large pores can be created by post-processing etching, the process is difficult to control accurately and may damage the mechanical strength of the microspheres.

[0004] In the field of heterogeneous catalysis, silica microspheres are often used as catalyst supports, which can highly disperse and stabilize active metal components (such as Pt, Pd, Au, etc.) on their surface. High specific surface area helps to increase the number of active sites, while open and through pore structure is conducive to the rapid diffusion of reactant and product molecules, reducing mass transfer limitations, thereby improving reaction rate and catalyst utilization. Traditional solid silica microspheres have too low specific surface area, which is not a good choice for carriers. Therefore, researchers have developed template methods (such as soft template and hard template methods) to prepare mesoporous silica materials with high specific surface area, such as MCM-41, SBA-15, etc. Such materials have regular and ordered nanochannels and extremely high specific surface area (up to 1000 m² / g or more), which greatly promotes catalytic reactions. However, they usually exist in the form of amorphous powder, and the particle size and morphology are difficult to control. When this powder material is directly used to fill the catalytic reaction bed, it will produce extremely high back pressure, and it is easy to be pulverized and lost due to airflow impact, which faces great engineering difficulties in actual industrial applications. The additional bonding and forming step is required to process it into macroscopic particles or spheres, which often leads to pore blockage, greatly reducing the advantage of high specific surface area.

[0005] In recent years, in order to balance the macroscopic morphology and the microscopic structure, researchers have tried various strategies. For example, nanosilica particles are agglomerated into micron-sized spheres by microemulsion method or spray drying method. However, these methods often have the following problems: (1) the organic solvents or surfactants used are expensive and difficult to remove; (2) the internal structure of the microspheres formed is disordered, the pore connectivity is poor, and the mechanical strength is weak; (3) the process is complex, the repeatability is poor, and it is difficult to scale up production.

[0006] Therefore, developing a new type of silica material that can cleverly combine the macroscopic microsphere morphology and the microscopic porous structure, that is, a microsphere with abundant mesoporous channels inside, which is self-assembled from nanostructured units by a bottom-up approach, has become an important research direction in this field. This material is expected to simultaneously solve the mass transfer efficiency problem in chromatographic separation and the carrier forming difficulty in catalytic application, and has great practical application value. SUMMARY

[0007] One of the purposes of the present application is to overcome the shortcomings of the prior art and provide a silica microsphere self-assembled from silica nanospheres, which has a large number of small pores inside.

[0008] The second purpose of the present application is to provide a preparation method of the above-mentioned silica microsphere. The method is simple in process, mild in conditions, low in cost, and easy to scale up production.

[0009] The third object of the present application is to provide the use of the above-mentioned silica microspheres in the preparation of chromatographic separation fillers and CO catalytic oxidation catalysts.

[0010] To achieve the above object, the present application adopts the following technical solutions: A silica microsphere, which is self-assembled from silica nanospheres as basic structural units, and a large number of fine channels formed by stacking between the nanospheres exist inside the microsphere. The diameter of the microsphere is 0.5-15 μm, and the diameter of the silica nanospheres constituting the microsphere is 10-250 nm. The BET specific surface area of the microsphere is 80-400 m 2 / g, the average pore size is 5-20 nm, and the total pore volume is 0.3-1.2 cm 3 / g.

[0011] A preparation method of the above-mentioned silica microspheres, comprising the following steps: (1) A certain amount of solvent and ammonia water is added to a round-bottom flask, then a certain amount of PEG4000 is added, and the solution is fully stirred to mix uniformly, then a certain amount of tetraethyl orthosilicate (TEOS) is quickly added, and after constant temperature stirring for a certain time, precursor A is obtained; (2) The precursor A is transferred into a stainless steel reaction kettle with a polytetrafluoroethylene lining, then the reaction kettle is placed in an oven, and kept at a certain temperature for a certain time, and then the obtained product is evaporated to remove the solvent to obtain silica nanospheres; (3) A certain amount of silica nanospheres prepared in step (2) and urea are dispersed into deionized water, hydrochloric acid or formic acid solution is used to adjust the pH of the system to ≤3.0, then a certain amount of formaldehyde aqueous solution is added, and after constant temperature stirring for a certain time, precursor B is obtained; (4) The precursor B is transferred into a stainless steel reaction kettle with a polytetrafluoroethylene lining, then the reaction kettle is placed in an oven, and kept at a certain temperature for a certain time, and then the product is cooled, centrifuged, washed and dried, and then calcined at a certain temperature for a certain time to obtain the porous silica microspheres.

[0012] Preferably, the solvent in step (1) is one or a mixture of two of isopropyl alcohol, ethylene glycol, n-propanol, n-butanol, methanol, ethanol, and water.

[0013] Preferably, the concentration of ammonia water in step (1) is 25-28wt%.

[0014] Preferably, the constant temperature stirring temperature in step (1) is 30°C, and the stirring reaction time is 6 hours.

[0015] Preferably, in step (2), the reaction is heated in an oven at 80-120°C for 12-24 hours.

[0016] Preferably, in step (3), the reaction is stirred at a constant temperature of 15-30°C for 3 hours.

[0017] Preferably, in step (4), the reaction is heated in an oven at 80-120°C for 12-24 hours.

[0018] Preferably, in step (4), the temperature is raised to 550°C at a rate of 2°C / min, and the calcination is performed for 4 hours.

[0019] The application of the above-mentioned silica microspheres in the preparation of chromatographic separation fillers and CO catalytic oxidation catalysts.

[0020] The silica microspheres are self-assembled from silica nanospheres with a diameter of 10-250 nm as basic units, and the overall diameter of the microspheres is 0.5-15 μm. A large number of small pores exist inside the microspheres, and the microspheres have the characteristics of high specific surface area (80-400 m² / g), suitable pore size (5-20 nm), and large pore volume (0.3-1.2 cm³ / g). 3 The preparation method comprises the following steps: firstly, synthesizing silica nanospheres through hydrolysis combined with a solvothermal method; secondly, assembling the nanospheres into microspheres through a secondary solvothermal process by using the polymerization reaction of urea-formaldehyde under acidic conditions as an inducing force; and finally, removing organic substances through calcination to obtain the final product.

[0021] The application has the following beneficial effects: 1. Unique structure: The silica microspheres provided by the application are a kind of multi-level structure material. The macroscopic morphology is a regular micron-level sphere, which is easy to fill and operate. The microscopic structure is self-assembled from nanospheres, and a rich, three-dimensional mesoporous channel is formed inside, which has a high specific surface area and a large pore volume. This structure is beneficial to the rapid mass transfer of reactants and products, and reduces the mass transfer resistance.

[0022] 2. Excellent performance: The high specific surface area and open pore structure, as well as the low metal impurity content make the silica microspheres have the characteristics of high column efficiency when used as chromatographic fillers; when used as catalyst carriers, the silica microspheres can highly disperse active components and provide sufficient reaction sites.

[0023] 3. Simple and green preparation method: The preparation method adopts a step-by-step solvothermal method, does not require expensive templates, and the urea-formaldehyde system used can be polymerized in situ under acidic conditions to assemble the silica nanospheres into microspheres as an adhesive, and is removed through subsequent calcination, without harmful substance residues, and is environmentally friendly. The process flow is simple, the conditions are easy to control, and the method is suitable for large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1is a scanning electron microscope (SEM) photo of the silica nanospheres prepared in Example 1 of the present application.

[0025] Figure 2 is a scanning electron microscope (SEM) photo of the silica microspheres prepared in Example 1 of the present application.

[0026] Figure 3 is a scanning electron microscope (SEM) photo of the silica microspheres prepared in Example 2 of the present application. DETAILED DESCRIPTION

[0027] The present application will be described in detail below with specific examples, but the scope of the present application is not limited thereto.

[0028] Example 1 (1) Preparation of silica nanospheres: 75 mL of anhydrous ethanol, 25 mL of anhydrous methanol and 5 mL of concentrated ammonia water (25 wt%) were added to a 250 mL round bottom flask, and then 0.05 g of PEG4000 was added and stirred at 30°C for 30 minutes to make it fully dissolved. Then 3 mL of TEOS was quickly added, and the reaction was continued at 30°C for 6 hours to obtain a milky white precursor A. 70 mL of the precursor A was transferred into a 100 mL polytetrafluoroethylene-lined stainless steel reactor, and heated in an oven at 120°C for 12 hours. After cooling, the obtained product was rotary evaporated at 60°C to remove most of the solvent and ammonia, to obtain a silica nanosphere dispersion liquid, which was dried to obtain a white powder.

[0029] (2) Preparation of silica microspheres: 0.8 g of the silica nanospheres prepared above and 3.0 g of urea were dispersed in 50 mL of deionized water and ultrasonicated for 30 minutes. Under stirring, 5 mol / L hydrochloric acid solution was added dropwise to adjust the pH value of the system to 1. Then 5.6 mL of 37 wt% formaldehyde aqueous solution was added, and the reaction was continued at 30°C for 3 hours to obtain a precursor B. The precursor B was transferred into a 100 mL polytetrafluoroethylene-lined stainless steel reactor, and heated in an oven at 120°C for 12 hours. After cooling, the product was collected by centrifugation, washed with water and ethanol three times each, and dried at 60°C for 12 hours. Finally, the dried powder was placed in a muffle furnace, and heated at a rate of 2°C / min to 550°C, and calcined for 4 hours to remove organic matter, and then naturally cooled to obtain silica microspheres.

[0030] The prepared silica nanospheres had a diameter of about 30 nm (as shown in Figure 1). Figure 1 The prepared silica microspheres had a diameter of about 2-3 μm (as shown in Figure 2). Figure 2), which is assembled by primary nanospheres with a diameter of about 30 nm, and has a large number of nanopores inside. The BET specific surface area of the silica microspheres is 140 m 2 / g, the average pore size is 12.5 nm, and the total pore volume is 0.45 cm 3 / g.

[0031] Example 2 (1) Preparation of silica nanospheres: 50 mL of anhydrous ethanol, 50 mL of anhydrous methanol and 5 mL of concentrated ammonia water (25 wt%) were added into a 250 mL round-bottom flask, and then 0.01 g of PEG4000 was added and stirred at 30°C for 30 minutes to make it fully dissolved. Then 4 mL of TEOS was quickly added, and the reaction was continued at 30°C for 6 hours to obtain a milky white precursor A. 70 mL of the precursor A was transferred into a 100 mL polytetrafluoroethylene-lined stainless steel reactor, and heated in an oven at 80°C for 24 hours. After cooling, the obtained product was rotary evaporated at 60°C to remove most of the solvent and ammonia, to obtain a silica nanosphere dispersion, which was dried to obtain a white powder. Silica nanospheres with a diameter of about 50 nm were obtained.

[0032] (2) Preparation of silica microspheres: 1.3 g of the silica nanospheres prepared above and 1.5 g of urea were dispersed in 25 mL of deionized water and ultrasonicated for 30 minutes. Under stirring, 5 mol / L hydrochloric acid solution was added dropwise to adjust the pH value of the system to 1.5. Then 2.8 mL of 37 wt% formaldehyde aqueous solution was added, and the reaction was continued at 15°C for 3 hours to obtain a precursor B. The precursor B was transferred into a 100 mL polytetrafluoroethylene-lined stainless steel reactor, and heated in an oven at 80°C for 24 hours. After cooling, the product was centrifuged and collected, washed with water and ethanol three times each, and dried at 60°C for 12 hours. Finally, the dried powder was placed in a muffle furnace, and heated at a rate of 2°C / min to 550°C, and calcined for 4 hours to remove the organic matter, and then naturally cooled to obtain silica microspheres.

[0033] Characterization showed that the prepared silica microspheres had a diameter of about 3-5.5 μm, which was assembled by primary nanospheres with a diameter of about 50 nm. The BET specific surface area of the silica microspheres was 82.5 m 2 / g, the average pore size was 21.8 nm, and the total pore volume was 0.43 cm 3 / g.

[0034] Application Example 1: As a chromatographic packing Take 2 g of silica microspheres prepared in Example 2 into a 100 mL flask with reflux device, add 100 mL of 15 wt% hydrochloric acid aqueous solution, and activate at 60°C for 5 hours. Wash the above product with deionized water to neutral, and vacuum dry at 100°C for 5 hours. Put 1 g of activated silica microspheres, 2 g of dimethyloctadecylchlorosilane, and 1 mL of 2,6-dimethylpyridine into 20 mL of toluene, and react at 100°C for 10 h. After filtration and toluene washing, the product is again treated with 100°C vacuum drying for 5 h. Then put the above product into a round-bottom flask, add 0.5 mL of trimethylchlorosilane and 15 mL of toluene solvent, and reflux at 45°C for 6 hours to complete the tailing operation. The product is again filtered, washed with toluene, and treated with 100°C vacuum drying for 5 h to prepare a chromatographic packing material.

[0035] A high-pressure homogenate technology is used to fill a 4.6 mm x 150 mm stainless steel chromatographic column. A mixture of uracil, nitrobenzene, and fluorene is used as a test sample, methanol / water (85 / 15, v / v) is used as the mobile phase, and separation tests are carried out at a flow rate of 0.9 mL / min. The results show that the three substances are baseline separated, and the chromatographic peak shape is symmetrical and sharp, and the theoretical plate number is high, indicating that the material has excellent separation performance and high column efficiency as a reversed-phase chromatographic packing material. The specific chromatographic evaluation parameters are as follows: Application Example 2: as a CO catalytic oxidation catalyst carrier A Au / SiO2 catalyst is prepared by deposition-precipitation method. Take 2 g of silica microspheres prepared in Example 1 into 50 mL of deionized water, and ultrasonically disperse for 10 min. Then add 2.07 mL of chloroauric acid aqueous solution (0.1 mmol / L) at one time, and magnetically stir at room temperature for 30 min. Then add NaOH (1 mol / L) dropwise to adjust the pH of the mixture to 10, and continue to stir for 1 h. After washing by centrifugation until no chloride ions are detected in the filtrate, the Au / SiO2 catalyst is dried at 60°C, and the target loading is 2 wt% Au.

[0036] Take 75 mg of the catalyst, and perform CO catalytic oxidation reaction evaluation in a fixed bed reactor. Before evaluation, the catalyst is first subjected to in-situ pre-reduction operation, using 10% H2-90% N2 mixed gas as the reducing gas, the heating rate is 5 ℃ / min, the reduction temperature is 500 ℃, and the reduction time is 1 h, and the temperature is naturally reduced to room temperature under the protection of the above reducing gas. The reaction gas composition is 1% CO, 10% O2, and the balance is N2, and the empty body flow rate is 20 mL / min. The results show that the CO complete conversion temperature of the catalyst is 35 ℃, which shows excellent low-temperature CO oxidation catalytic activity, which is due to the high specific surface area of the carrier, the high dispersion of Au nanoparticles in the open pores, and the good mass transfer of the reactants CO and O2.

[0037] The above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be within the scope of the present application.

Claims

1. A silica microsphere, characterized in that, The microspheres are constructed by the self-assembly of silica nanospheres. The diameter of the microspheres is 0.5–15 μm, the diameter of the nanospheres is 10–250 nm, and the specific surface area of ​​the microspheres is 80–400 m². 2 / g, with an average pore size of 5~25 nm and a total pore volume of 0.3~1.2 m³. 3 / g.

2. A method for preparing silica microspheres as described in claim 1, characterized in that, Includes the following steps: (1) Add a certain amount of solvent and ammonia to a round-bottom flask, then add a certain amount of PEG4000, stir thoroughly to make the solution evenly mixed, then quickly add a certain amount of tetraethyl orthosilicate, stir at a constant temperature for a certain time to obtain precursor A; (2) The precursor A was then transferred into a stainless steel reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and kept at a certain temperature for a certain time. The product was evaporated to remove the solvent and then silica nanospheres were obtained. (3) Weigh a certain amount of silica nanospheres and urea and disperse them in deionized water. Adjust the pH of the system to ≤3.0 using hydrochloric acid or formic acid solution. Then add a certain amount of formaldehyde aqueous solution and stir at a constant temperature for a certain time to obtain precursor B. (4) The precursor B is transferred into a stainless steel reactor with a polytetrafluoroethylene liner. The reactor is then placed in an oven and kept at a certain temperature for a certain time. After cooling, the product is centrifuged, washed, and dried. It is then calcined at a certain temperature for a certain time to obtain silica microspheres.

3. The preparation method according to claim 2, characterized in that, The solvent mentioned in step (1) is one or a mixture of two of isopropanol, ethylene glycol, n-propanol, n-butanol, methanol, ethanol, and water.

4. The preparation method according to claim 2, characterized in that, The concentration of ammonia in step (1) is 25~28wt%.

5. The preparation method according to claim 2, characterized in that, In step (1), the constant temperature stirring temperature is 30℃ and the stirring reaction time is 6 hours.

6. The preparation method according to claim 2, characterized in that, In step (2), the reaction is carried out in an oven at 80-120℃ for 12-24 hours.

7. The preparation method according to claim 2, characterized in that, In step (3), the reaction is carried out at a constant temperature of 15-30℃ with stirring for 3 hours.

8. The preparation method according to claim 2, characterized in that, In step (4), the reaction is carried out in an oven at 80-120℃ for 12-24 hours.

9. The preparation method according to claim 2, characterized in that, In step (4), the temperature is increased to 550°C at a rate of 2°C / min and calcined for 4 hours.

10. The application of the silica microspheres as described in claim 1 in chromatographic separation packing materials and CO catalytic oxidation catalyst supports.

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