JPH0261425B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing porous glass, and more particularly to a method for producing porous glass by treating a lump of phase-separated alkali borosilicate glass with acid. Conventionally, alkali borosilicate glass is phase-separated into a silica phase and an alkali borate phase by heat treatment, and the alkali borate phase is eluted by acid treatment to obtain porous glass, and the heat treatment conditions are adjusted to obtain a phase-separated state. It is well known that porous glass having a desired pore size can be obtained by controlling the pore size, and this porous glass is being used as a carrier for catalysts and enzymes, molecular sieves, gas separation, and the like. However, porous glass is mainly used in the form of fine powder, and therefore its scope of use is extremely limited. One reason for this is that in the case of bulk porous glass, the acid treatment process requires a considerable amount of time and cracks are likely to occur, making it extremely difficult to manufacture in practice. Furthermore, the pore diameter of porous glass is usually measured by electron microscopy, but the actual value of its surface area (meaning the surface area per unit weight (m ² /g)) can be predicted from the pore diameter. Similarly, the measured value of pore volume (pore volume per unit weight (cc/g)) is significantly lower than the predicted value.
This is because the alkali boric acid phase that should be leached out by the acid contains about 10% of silica, which remains in the pores in the form of a colloid without being leached out by the acid. This is schematically shown in Figure 1. Since this silica colloid is scattered and deposited within the pores, the actual pore diameter differs from one individual to another, which is a serious hindrance in its use as a carrier for enzymes, molecular sieves, and gas separation.
There is a trend that the higher the surface area value is, the better it is in terms of adsorption performance, but the actual measured value includes the surface area of the silica colloid mentioned above, so it is not necessarily appropriate, and is rather suitable for the above uses. In this case, it is essential that the pore diameter be uniform. By the way, if the glass is in the form of fine powder or an extremely thin plate with a thickness of several hundred microns, the silica colloid can be dissolved and removed by dipping it in an acid and then dipping it in a dilute alkaline aqueous solution, and the original pore size can be obtained. However, when this method is applied to lumps, it has been a problem because not only the silica colloid but also the silica layer remaining as a skeleton gradually dissolves and collapses from the surface layer. The present invention improves these conventional techniques and solves their problems, and provides a particularly suitable method for producing bulk porous glass. Specifically, the present invention provides a suitable method for producing bulk porous glass, in which phase-separated alkali borosilicate glass is acid-treated to form porous glass. The gist of the method is a method for producing porous glass, which is characterized in that it is first treated with an aqueous solution of a strong acid of 0.1 to 0.5N, and then treated with an aqueous solution of a strong acid of 2 to 5N. . The lump glass applied to the present invention has a thickness of 1 mm.
The above plate-like bodies, similar rectangular bodies, prismatic bodies, etc.
and spheres with a diameter of 1 mm or more, similar elliptical bodies, cylindrical bodies, etc., and combinations thereof. In the present invention, phase-separated alkali borosilicate glass is first immersed in an aqueous solution of 0.1 to 0.5 N strong acid to remove the alkali boric acid phase. The alkali borosilicate glass may be of a commonly used and known composition. The strong acid is sulfuric acid, nitric acid, hydrochloric acid, or a mixed acid thereof; other mineral acids or organic acids are not preferred because they increase the processing cost. When the acid concentration is less than 0.1N, the elution of the alkali boric acid phase becomes extremely slow. When the acid concentration exceeds 0.5N, the elution of the alkali boric acid phase is rapid, and the volume of the remaining silica phase that forms the skeleton also changes rapidly, creating stress between it and the unreacted material, resulting in a crack. tends to occur. A more important reason lies in the fact that dilute acids are more likely to elute silica. The present inventor investigated the behavior of silica, which is present in a considerable amount in the alkali boric acid phase, toward acid and found that the dilute the acid, the more the silica gradually dissolves and becomes small droplets (Figure 2). ). Note that the relatively dense silica phase remaining as a skeleton is also slightly dissolved, but to a negligible extent. On the other hand, in the case of a concentrated acid, almost no silica is dissolved and the silica aggregates. The amount of silica dissolved increases in acids below 0.5N, and is dissolved in balance with the dissolution of the alkaline borate phase.
Significant in 2N to 5N acids. In addition, the concentration of acid −
Regarding the relationship between silica dissolution, THElmer et al. (Journal of The American Ceramic Society,
53 (1970)) that the lower the concentration of nitric acid, the greater the amount of silica dissolved. It is quite natural from chemical equilibrium theory that if the amount of acid is small, the concentration of alkali borate and silica in the acid solution will increase during the elution process and the dissolution ability will decrease. Therefore, the larger the amount, the better, but from a practical standpoint, the ratio of acid amount (cc)/glass weight (g) is preferably 200 to 800. The acid treatment temperature is usually used, and a temperature at which the reaction is active, that is, around 100°C, is appropriate. The acid treatment time is initially determined by visual observation. That is, during the elution process, the boundary area between the eluted phase and the uneluted phase can be clearly seen, and the time when the boundary area disappears is the completion time. Hereinafter, if glass lumps of the same shape are treated under the same processing conditions and for the same processing time, reproducible results will be obtained. Typically for glass blocks with a thickness of 1 to 10 mm, the time range is from 36 to 420 hours. The primary treated glass thus obtained is further treated with an aqueous solution of a 2N to 5N strong acid. In addition,
Since most of the alkali boric acid phase is eluted in the primary treatment, cracks will not occur in the glass even if the acid concentration is rapidly increased in the secondary treatment. It can be confirmed that in this secondary treatment, the surface area of the primary treated glass decreases and the pore volume increases. As mentioned above, this is because the colloidal silica remaining in the pores, scattered and deposited is not dissolved in the acid, but aggregates with each other, reducing the surface area (Figure 3), while the surface area is still not fully dissolved. The pore volume is increased due to the dissolution of the alkali boric acid phase that is not present. Note that when a concentrated acid exceeding 5N is used, the pore volume hardly increases, but the reason for this has not been elucidated. Moreover, if it is less than 2N, aggregation becomes slow. The amount of acid is the volume of acid (cc)/weight of glass (g).
It is in the range of 20 to 80, and even if it exceeds 80, the above-mentioned effect will not be achieved, and if it is less than 20, the effect will be small. The acid treatment temperature is usually used, and a temperature at which the reaction is active, that is, around 100°C, is appropriate. The acid treatment time is determined based on changes over time in the weight, pore volume, or surface area of the glass. That is, during the treatment process, the weight and surface area of the glass gradually decrease, and the pore volume gradually increases, but eventually these fluctuations become extremely small. This point may be considered as the completion time. Upon completion of this process, the pore volume reached approximately 80% of the pore volume of the porous body obtained by separately treating finely powdered glass with an acid-alkali treatment, that is, the target value, as described above, and the pore size Since it is uniform, it can be sufficiently used as a carrier for the enzymes and the like mentioned above. Typical processing times are from 24 hours to 360 hours for bulk glass with a thickness of 1 to 10 mm. The method for producing porous glass from alkali borosilicate glass has been described above, but this production method uses alkali borosilicate glass with a fourth component such as titania,
Of course, glass with added zirconia, phosphoric acid, etc.
It can also be applied to phosphosilicate glass, aluminophosphate glass, etc. Hereinafter, the present invention will be explained in detail with reference to specific examples. _SiO2 66.1wt%, _{Al2O3 1.6wt} %, _{B2O3 24.1wt} %,
Na ₂ O 8.2wt%, AS ₂ O ₃ 0.3wt%, NaNO ₃ 0.5wt%
After producing soda borosilicate glass consisting of the following, it was heat-treated at 640°C for 48 hours to separate the phases, and then a large number of blocks with a width of 40 mm, a length of 60 mm, and a thickness of 3 mm were produced and subjected to acid treatment. In order to know the pore structure of this glass, the pore diameter, pore volume, and surface area were measured in advance by the known method described above. That is, a part of this glass was made into a powder (0.8 to 1.0 mmφ) and placed in a 1N sulfuric acid aqueous solution at 100℃.
The sodium borate phase was removed by immersion for 72 hours, and then the colloidal silica in the pores was removed by immersion in a 0.5N caustic potassium aqueous solution at room temperature for 6 hours. The pore diameter was observed using an electron microscope, the pore volume was determined by measuring the difference between the weight of the sample powder at saturated water absorption and its dry weight, and the surface area was measured using an automatic specific surface area measuring device based on the so-called BET method. Obtained target value. Pore diameter: average 500 Å Pore volume: 0.6 cc/g Surface area: 50 m ² /g The primary and secondary acid treatment conditions were changed as appropriate,
The pore diameter, pore volume, and surface area of the obtained porous glass were measured using the measurement methods described above. The results are shown in Table 1. In Examples 1 to 4 of the present invention, the pore volume is about 80% and the surface area is about 150 to 160% of the target values described above, which are almost satisfactory values, and the pore diameters are uniform. Therefore, it can be fully used as a carrier for enzymes, molecular sieves, and gas separation.

[Table] Comparative Examples 1, 2 and 3 are dilute acid,
This experiment was carried out in comparison with Example 1 regarding concentrated acids and extremely concentrated acids. That is, Comparative Example 1 had the same primary treatment conditions as Example 1. However, only the processing time was the same as the total processing time of Example 1. Comparative Example 2 has the same secondary treatment conditions as Example 1. However, only the processing time was the same as the total processing time of Example 1. Comparative Example 3 has a higher acid concentration than Comparative Example 2. Comparative Example 1 did not have the same effect as the present example, and Comparative Examples 2 and 3 exhibited cracks. In Comparative Examples 4 and 5, a primary treatment with a dilute acid and a secondary treatment with a concentrated acid were performed in comparison with Example 1. In Comparative Example 4, when the acid concentration in the secondary treatment was too high, Comparative Example 5 also shows the case where the acid concentration is too low. Both Comparative Examples 4 and 5 did not have the same effect as the present example. As detailed above, the present invention enables the production of bulk porous glass, which has been considered difficult in the past, and since the pore diameters are uniform, it can be used as a carrier for enzymes, molecular sieves, etc.
It can be put to practical use for gas separation, etc.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view showing the state of the pores in a conventional porous glass, FIG. 2 is a schematic plan view showing the state of the pores in the porous glass after the primary treatment of the present invention, FIG. 3 is a schematic plan view showing the state of pores in the porous glass of the present invention. 1... Silica skeleton, 2... Pore, 3... Colloidal silica.

Claims (1)

[Claims] 1. In the method of producing porous glass by acid-treating phase-separated alkali borosilicate glass, first
with an aqueous solution of 0.1-0.5N strong acid, then 2-5N
A method for producing porous glass, characterized in that the acid treatment is carried out using an aqueous solution of a strong acid.