METHOD FOR PREPARING MESOPOROUS SILICA AND THE MESOPOROUS SILICA THEREFROM

[Technical Field]

The present invention relates to a method for preparing mesoporous silica and the mesoporous silica obtained therefrom, specifically, to a method for preparing mesoporous silica having a high specific surface area by using a suitable surfactant in preparing gel-type silica and applying optimized conditions for drying and firing processes, and the resulted mesoporous silica therefrom. Mesoporous silica of the present invention may have various uses, such as heat insulators for a high temperature use, adsorbents for a high temperature use, catalysts, catalyst carriers or the like.

[Background Art]

According to high-technological development in various industries, metal and non-metal materials having a high specific surface area have been in increasing demand. Under such circumstances, active researches are made to prepare such materials in efficient way. As one of such materials, silica is generally used in many industrial fields, and particularly, mass-production of silica having a high specific surface area in cost-effective way has still been demanded in various fields.

Silica is a material expected to be a good adsorbent.

Currently, the most generally used adsorbent is an activated carbon, however its use in an incinerator or combustion furnace, or a process involving exhaust of a high temperature

gas, has been limited, since carbons, the major component of activated carbon, are very rapidly combusted at a temperature in the range of 600-700 ^° C or more. On the contrary, porous silica is able to maintain the silica particles as well as their porosity, even at very high temperature such as 1000 ^° C or more, thereby being possibly used as an effective substitutive material for activated carbon.

Particularly, mesoporous silica is known to have a significantly high heat insulating property, thereby being possibly used as an insulation material in various fields, and its many pores also allow its use as a catalyst carrier. However, the high production cost for such porous silica has restricted its general use.

As for methods for producing silica with reduced cost, a method which uses water glass as a starting material and a sol-gel process, has been generally known. However, when moisture is present during a drying process, particle agglomeration is occurred, and the silica particles resulted from said sol-gel process are provided in the form of hard agglomerates, thereby having a small specific surface area such as tens of m ² /g. Further, silica gel which is made to have a high specific surface area and used as a desiccant, only has a specific surface area of around 300-400m ² /g.

[Disclosure]

[Technical Problem]

The purpose of the present invention is to provide a method for preparing mesoporous silica which has nano-size pores and a high specific surface area, by using cost- effective water glass as a starting material as well as mesoporous silica obtained therefrom.

[Technical Solution]

According to a first aspect of the present invention, provided is a method for preparing mesoporous silica, comprised of: a) providing a mixture of 0.5-0.9N HCl and a surfactant; b) preparing silica gel by adding water glass (sodium sillicate) to the mixture of the previous step and adjusting the pH value to 5-7 at a temperature of 30-50 ⁰ C; c) removing impurities from the resulted silica gel by washing it with distilled water and filtration; d) drying the resulted silica with a microwave heating device; and e) firing the dried silica at a temperature of 400°C or more.

In the above method for preparing mesoporous silica, the surfactant may be polyethylene glycol, hydroxypropyl cellulose or mixtures thereof. The surfactant may be added to the amount 1.5-10wt%, based on the weight of the finally obtained silica particles, and the molecular weight of the polyethylene glycol may be 6,000-35,000. In the mixture of polyethylene glycol and hydroxypropyl cellulose, the mixing ratio of said two components, based on the weight, may be 1:1, and then the resulted mixture is used.

According to another aspect of the present invention, mesoporous silica particles prepared by the forgoing method are provided. The silica is provided in the form of a powder, which may have an average particle size of 20-4OjMm and a specific surface area of 700-900m ² /g. [Advantageous Effects]

The present invention has some benefits and advantages. The mesoporous silica powder according to the present invention has a very high specific surface area such as 700- 900m ² /g, thereby being suitably used as a substitute for porous materials such as activated carbon. Further, it is still very stable at a high temperature such as 1000 ^° C or more, thereby being suitably used in various high technology industries as heat insulators for a high temperature use, adsorbents for a high temperature use, catalysts or the like. Since the method of the present invention uses inexpensive water glass as a starting material, it is possible to provide mesoporous silica particles in an economical and efficient way.

[Description of Drawings]

Fig. 1 is a schematic illustration showing the effect of a surfactant on particles.

Fig. 2 is a plot showing the relation between ξ potential of the particle surface and pH of a solution, when a surfactant is added.

Fig. 3 is a plot comparing the specific surface areas of the resulted silica relative to the type and amount of a surfactant added thereto.

Fig. 4 is a plot comparing the specific surface areas of the resulted silica relative to a drying method.

Fig. 5 is a plot comparing the specific surface areas of the resulted silica relative to the molecular weight of an added surfactant, PEG.

Fig. 6 is a particle size distribution of silica powders prepared by one embodiment of the present invention. Fig. 7 is a bar graph showing the specific surface area

of silica powders prepared by one embodiment of the present invention.

Fig. 8 is a SEM (scanning electron microscope) photo of silica powders prepared by one embodiment of the present invention.

[Mode for invention]

Hereinafter, a method for preparing mesoporous silica of the present invention is further illustrated step by step. The present invention, characteristically prepares mesoporous silica particles in which minute pores in nano- size are formed, by adding a surfactant which agglomerates nano-sized particles to a proper extent and applying optimized drying and firing processes. The term "mesoporous" used herein refers to a state in which minute pores in nano-size are formed, and is generally regarded as same as the term "nanoporous".

According to one embodiment of a method for preparing mesoporous silica of the present invention, firstly a mixture of a surfactant and 0.5-0.9N HCl is prepared. As for the surfactant used in the present invention, preferred is PEG

(Polyethylene glycol), HPC (Hydroxy propyl cellulose) or mixtures thereof. When using a mixture of PEG and HPC, they are preferably mixed in the same weight ratio. The molecular weight of PEG used is preferably between 6,000 and 35,000, particularly preferably 20,000. The molecular weight of HPC is not specifically restricted.

Each of HPC and PEG is mixed with HCl in an amount of 1.5wt%-10wt%, based on the weight of the finally obtained silica particles. Most preferably, each of HPC and PEG is used in an amount of 2.5wt%, based on the weight of the

finally obtained silica particles, in order to obtain the highest specific surface area (see, Fig. 3).

The surfactant used in the present invention is served to increase the specific surface area by surrounding silica particles and securing spaces among the silica particles, as shown in Fig. 1. Since the silica particles are surrounded by surfactant as such, it is believed that agglomeration among the particles is significantly prevented, even if a drying process is carried out in the presence of moisture. The mixture of a surfactant and HCl as prepared above, is added to purified water glass at 30-50 ^° C with stirring, and the pH value of the resulted mixture is adjusted to 5-7 so as to form silica precipitates in gelled state. When the temperature is 30 ^° C or less, gellation of silica proceeds too slowly, and when the temperature is 50 ^° C or more, gellation is carried out so rapidly that the gel tends to be hardened. In the meantime, purification of water glass used herein may be carried out by conventionally known methods in this field of art. In the mixing process using water glass, the surfactant and HCl should be in the form of a previously prepared mixture. If a surfactant is added alone, without being previously mixed with a HCl solution, to water glass, it is not possible to obtain mesoporous silica particles, and to increase the specific surface area to more than lOOmVg, although agglomeration of particles to the particle size of several tens of μm is prevented.

In the meantime, the reason for adjusting the pH value to 5-7 in silica gel preparation of the present invention is to lower ξ potential. Fig. 2 is a plot representing ξ potential of silica particles measured when a surfactant is

added. In the plot, it can be found that the ξ potential is lowest in the pH range of 5-7, meaning that agglomeration among particles occurs in only small degree. Therefore, it is believed that the mechanism of pore formation in silica of the present invention is that the silica particles in nano- size reduce moisture and ξ potential of the particle surface, wherein the moisture causes agglomeration of particles during a drying process, thereby allowing an appropriate degree of agglomeration to proceed in the presence of a surfactant which suppresses the agglomeration of particles, and then finally forming nano-sized pores.

Thus formed silica gel is washed and filtered with distilled water to remove other residual impurities such as NaCl, and then dried for removing residual moisture in the silica gel. According to the present invention, the drying process is carried out by using a microwave heating device, in which commonly used microwave may be used. As shown in Fig. 4, when using a microwave heating device for dry, the resulted specific surface area is higher than that obtained from conventional oven drying. It is believed that such higher specific surface area is caused by a popcorn-like effect in the microwave heating device.

Finally, the dried silica particles are fired at 400- 1000 ^° C for 4 hours or more to obtain mesoporous silica particles. At this time, when the firing process is carried out at more than 1000 ^° C, a reaction between components of the reactor vessel and silica occurs, causing phase transformation, for example resulting in a compound made of Trydimite and mullite. According to such phase transformation, particle structures and then a specific surface area become changed. It was confirmed that the

silica particles obtained as such had an average particle size of 20-40μm and a specific surface area of 700-900m ² /g with an electron microscope.

Since the mesoporous silica powder provided by the present invention has a specific surface area as high as 700-

900m /g, it can be used as a substitute for porous materials such as activated carbon. Further, it is stable under high temperature condition such as 1000 ^° C or more, therefore it may be effectively used in various high technology industries as heat insulators for a high temperature use, adsorbents for a high temperature use, catalysts or the like. Furthermore, since the present invention uses inexpensive water glass as a starting material, it is possible to provide mesoporous silica particles in an economical and efficient way. Hereinafter, the present invention is further illustrated in detail through the examples given below, without any intention to restrict the present invention.

EXAMPLES Example 1

A mixed solution of 150ml of 0.6N HCl, 0.3g of HPC and 0.3g of PEG having a molecular weight of 20,000 was added to

100ml of 20Be water glass at 40 ^° C to obtain a pH value ranged between 5-7, resulting in gel type nano-silica precipitates. Then, the precipitates were washed three times with distilled water, then once with 0.01N H ₂ SO ₄ and once again with distilled water, and then were filtered. Thus obtained product was dried in a microwave heating device for 10 minutes. Then, it was sufficiently fired at 400 ^° C for more than 4 hours. The resulted silica powder had an average particle size of 20-40μm (Fig. 6), and the specific surface

area was 840m ² /g (Fig. 7) .

Fig. 8 shows a SEM photo of the above-prepared silica particles. As seen from the photo, it can be confirmed that minute pores in about 50nm size have been developed well. Owing to such minute pores, the silica prepared according to the present invention can provide highly added value and functions .

Examples 2-4 Silica particles were prepared by the same method as in Example 1, except using PEG having a molecular weight of 6,000, 20,000 and 35,000, respectively, and then each specific surface area thereof was measured. As a result, all of the silica particles obtained by using PEG in such molecular weight range, had 700m /g or more of a specific surface area, and particularly PEG with a molecular weight of 20,000 provided silica particles having the highest specific surface area, as shown in Fig. 5.

Comparative examples 1-2

Silica particles were prepared by the same method as in Example 1, except using PEG having a molecular weight of 200 and 600, respectively, and then each specific surface area thereof was measured. The results were shown in Fig. 5. As it can be seen from Fig. 5, the specific surface area of each silica powder according to comparative examples 1 and 2 was 700m /g or less .

[industrial availability] The mesoporous silica powder provided by the present invention has a specific surface area as high as 700-90Om /g,

and is stable under high temperature condition such as 1000 ^° C or more, therefore it has various uses for example, as heat insulators for a high temperature use, adsorbents for a high temperature use, catalysts or the like. Furthermore, since the present invention uses inexpensive water glass as a starting material, it is possible to provide mesoporous silica particles in an economical and efficient way.