JPH09143184A - Production of high-purity tetramethoxysilane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high-purity tetramethoxysilane useful as a raw material for synthetic quartz powder or synthetic quartz molding, by subjecting crude tetramethoxysilane prepared by reacting metal silicon with methanol to superfractionation under a specific condition. SOLUTION: Crude tetramethoxysilane prepared by reacting metal silicon with methanol is subjected to superfractionation by making difference in temperature between the top and the bottom of a column <=20 deg.C to remove a high- boiling component. The crude tetramethoxysilane is obtained by using 1mol of metal silicon and 4-10mols of methanol and 1.5-10mol based on 1g of the metal silicon of dodecylbenzene as a reaction solvent. The superfractionation is usually carried out by using a distillation column installing 5-50 plate trays or packed materials while refluxing a part of a condensed liquid at the top of the column at 0.1-5 reflux ratio. The distillation and purification are preferably carried out to make content of trimethoxymethylsilane in tetramethoxysilane after the separation of a low-boiling component from the crude tetramethoxysilane <=0.15wt.%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing high-purity tetramethoxysilane useful as a raw material for synthetic quartz powder or a synthetic quartz molded body.

In recent years, in glass products used in the optical communication field, the semiconductor industry, etc., very strict control has been performed on the trace impurities. For example, when the glass product is a crucible for pulling up a silicon single crystal, if a trace amount of impurities is contained, the trace amount impurities eluted from the crucible are mixed into the silicon single crystal and the performance of the semiconductor is deteriorated. In particular, III-V group elements such as phosphorus and boron deteriorate the semiconductor characteristics, so that great care is required. Further, in the case of a quartz jig used at high temperature, III-V contained in the quartz molded body is used.
Since the group element diffuses and contaminates the silicon wafer, desired semiconductor characteristics cannot be obtained.

Such high-purity glass is mainly used for refining natural quartz, and for attaching and growing fumes generated by decomposition of silicon tetrachloride in an oxyhydrogen flame onto a substrate.
It is produced by, for example, a method using synthetic quartz powder obtained by firing silica gel obtained by hydrolysis and gelation of silicon alkoxide and the like.

However, the method (1) has a limit in reducing the content of trace impurities, and the method (2) has a problem that the manufacturing cost is extremely high. Further, the method using silica gel, particularly the method using silicon alkoxide as a raw material, can obtain a synthetic quartz powder having a relatively low content of trace impurities, but it cannot be said that the required level is necessarily satisfied.

[0005]

In view of the above situation, the present inventors have proposed a method of producing a synthetic quartz powder or a synthetic quartz compact having a phosphorus content lower than that of a conventional method by using a silicon alkoxide as a raw material. As a result of intensive studies, the inventors have found that phosphorus in the raw material silicon alkoxide, which is the main cause of these problems, can be significantly reduced under specific distillation conditions, and completed the present invention.

[0006]

That is, the present invention provides a method for purifying crude tetramethoxysilane obtained by the reaction of metallic silicon and methanol to produce high-purity tetramethoxysilane, which is higher than crude tetramethoxysilane. In separating the boiling components, there is a method for producing high-purity tetramethoxysilane, which is characterized in that the temperature difference between the tower top and the tower bottom is set to 20 ° C. or less and precision distillation is performed.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The crude tetramethoxysilane used in the present invention has the following (1)
As shown in the formula, it is obtained by reacting metallic silicon with methanol.

[0008]

[Equation 1] Si + ₄ MeOH → Si (OMe) ₄ + 2H ₂ (1)

The amount of methanol used is selected in the range of 3 to 50 times mol, particularly preferably 4 to 10 times mol, per mol of metallic silicon. When the amount is less than 3 times the molar amount, unreacted metallic silicon remains in a large amount and the productivity is low. On the other hand, if it exceeds 50 times by mole, a large amount of unreacted methanol remains, and a large amount of energy is required for separation and purification.

Although a hydrocarbon solvent can be widely used as a reaction solvent, a high boiling point solvent which is easily separated from tetramethoxysilane is suitable for use. As a high boiling point solvent,
Examples thereof include alkylbenzene compounds such as triethylbenzene, octylbenzene, dodecylbenzene and didodecylbenzene, and arylmethane compounds such as diphenylmethane, benzyltoluene and dibenzyltoluene. Of these, dodecylbenzene is particularly preferable because it has a large difference in boiling point from the target product, tetramethoxysilane, is easy to separate, and is relatively inexpensive. The amount of the solvent to be used is industrially suitably selected from the range of 1 ml to 100 ml, particularly 1.5 to 10 ml per 1 g of metallic silicon. This is because if the amount of the solvent is too small, the metallic silicon cannot be made into a slurry state, and the reaction cannot be performed. On the contrary, if the amount is too large, the productivity per unit is low and a large apparatus is required.

As a catalyst for this reaction, a basic catalyst such as an alkali alcoholate (Japanese Unexamined Patent Publication No. 52-12133)
And copper catalysts such as copper chloride (Japanese Patent Publication No. 50-34538)
It has been known. The reaction temperature varies depending on the solvent, the catalyst, etc., but is usually 100 to 300 ° C. The pressure during the reaction is a value obtained by adding the pressure of hydrogen gas generated by the reaction to the vapor pressure of methanol and hydrocarbons at the above reaction temperature. The reaction can also be performed.

In addition to the above one-step reaction, once a mixed composition of tetramethoxysilane, trimethoxymethylsilane and methanol is obtained, tetramethoxysilane is used as a catalyst with an oxide or hydroxide of an alkali metal or alkaline earth metal. A method for obtaining silane has also been proposed (JP-A-63-16).
6888).

In the reaction composition thus obtained (hereinafter referred to as "crude tetramethoxysilane"), in addition to the target substance tetramethoxysilane, a solvent, unreacted methanol, insoluble components and by-products. Which contains trimethoxymethylsilane and a silicon-containing oligomer.

In the present invention, high purity tetramethoxysilane is obtained by purifying such crude tetramethoxysilane. For efficient purification, for example, the following method may be mentioned. First, the insoluble component is separated. The insoluble component is dispersed as a solid in the reaction liquid, and can be separated by filtration or can be separated as a distillation residue together with a part of the high boiling component using a thin film evaporator. Since the insoluble component in the crude tetramethoxysilane often has poor filterability, the latter method is preferred for industrial use.

In the purification method after removing the insoluble component, first, a component having a lower boiling point than tetramethoxysilane is separated by distillation. The low boiling point component here is a component having a lower boiling point than tetramethoxysilane, such as unreacted methanol and trimethoxymethylsilane.

For distillation separation, an ordinary distillation apparatus can be used. The operation can be performed either batchwise or continuously. In order to precisely separate tetramethoxysilane from methanol and trimethoxymethylsilane, trays or packings are installed in the distillation column. The number of stages is 5 to 50,
The tower diameter is determined by the processing conditions. The pressure is usually normal pressure or reduced pressure. In order to perform the separation precisely, a part of the condensate at the top of the column is operated while being refluxed to the column,
The reflux ratio is usually 0.1-5. When the distillation is carried out batchwise, the operation may be carried out while changing the reflux ratio depending on the composition of the liquid flowing out. Further, in the batch operation, high-purity tetramethoxysilane will flow out after the low boiling point component, but in the continuous operation, the tetramethoxysilane will be extracted from the reboiler.

In the present invention, it is preferable to carry out distillation purification so that the content of trimethoxymethylsilane in tetramethoxysilane after separating the low-boiling component from crude tetramethoxysilane is 0.3% by weight or less. . The structure of the phosphorus compound having a lower boiling point than that of tetramethoxysilane is not clear, but when the content of trimethoxymethylsilane in tetramethoxysilane is more than 0.3% by weight, the concentration of phosphorus may increase sharply. It was confirmed by the experiments of the present inventors.

Next, components having a higher boiling point than tetramethoxysilane are separated by distillation. The high-boiling component mentioned here is a component having a higher boiling point than tetramethoxysilane such as a silicon-containing oligomer and a solvent.

For the distillation separation, an ordinary distillation apparatus can be used. The operation can be performed either batchwise or continuously. When the low-boiling component is separated by distillation by a batch operation, high-purity tetramethoxysilane can be allowed to flow out after the low-boiling component, and the high-boiling component can be separated by distillation with the same distillation apparatus.
On the other hand, when performing low-boiling component distillation separation in a continuous operation, tetramethoxysilane is extracted from the reboiler, and the extracted liquid is supplied to a distillation apparatus installed in the series to separate high-boiling components by distillation. You can

In order to precisely separate tetramethoxysilane from the silicon-containing oligomer, the solvent and the like, a tray or a packing is installed in the distillation column. The number of steps is 5 to 50
In a stage, the tower diameter is determined by the processing conditions. The pressure is usually normal pressure or reduced pressure. In order to carry out the separation precisely, a part of the condensate at the top of the column is operated while being refluxed in the column, and the reflux ratio is usually 0.1 to 5. When the distillation is carried out batchwise, the operation may be carried out while changing the reflux ratio depending on the composition of the liquid flowing out.

In the present invention, in separating the high boiling point component from the crude tetramethoxysilane, it is essential that the temperature difference between the top and bottom of the column is 20 ° C. or less and the distillation purification is performed. Although the structure of the phosphorus compound having a higher boiling point than tetramethoxysilane is not clear, it was confirmed by experiments that the concentration can be reduced to 10 ppb or less by distillation purification with the temperature difference between the column top and the column bottom being 20 ° C. or less. The temperature difference between the top and bottom of the tower is 2
Even if the temperature exceeds 0 ° C, the silicon-containing oligomer, the solvent and the like can usually be separated substantially completely, but if the temperature difference exceeds 20 ° C, the concentration of phosphorus sharply increases.

When the operation is carried out by the batch method, the temperature of the reboiler is measured, and the distillation is terminated when the temperature difference between the top and the bottom of the column reaches 20 ° C. On the other hand, when the operation is carried out by the continuous method, the temperature difference between the top and the bottom of the tower is controlled by controlling the concentration of tetramethoxysilane in the liquid in the reboiler.
It can be kept below ℃.

When the purification after removing the insoluble components is carried out by a continuous method, it is possible to first separate the high boiling components from tetramethoxysilane and then to separate the low boiling components. In this method, high-purity tetramethoxysilane is finally extracted from the reboiler, and trace metals are mixed from the device during operation. Therefore, it is desirable to perform distillation treatment again to achieve high purity.

By the above-mentioned production method of the present invention, it is possible to easily achieve the content of trimethoxymethylsilane in tetramethoxysilane of 0.3% by weight or less, preferably 0.15% by weight or less. Using tetramethoxysilane having a low content of trimethoxymethylsilane, when silica gel obtained by the sol-gel method is fired to produce synthetic quartz powder, the generation of black powder during firing of silica gel is greatly reduced, The quartz glass molded body produced by using such a quartz powder having a small amount of black powder also greatly reduces the generation of minute bubbles. Although the mechanism for this phenomenon has not been clarified, the usual methoxy group is practically eliminated by demethoxylation or oxidation during firing, but the methyl group is difficult to be eliminated and the oxidation rate is slow,
It is considered that unburned carbon is likely to remain in the synthetic quartz powder to generate black particles, which may cause bubbles during molding using such quartz powder.

On the other hand, phosphorus, which is a trace amount of impure component and has recently received the most attention, has been considered to be contaminated from the atmosphere until now. It was found to be mixed in from methoxysilane. That is, a very good positive correlation is found between the content of phosphorus in tetramethoxysilane and the content of phosphorus in the synthetic quartz powder or its molded product. Although the form of phosphorus in tetramethoxysilane has not been clarified, it is considered to be an organophosphorus compound produced by the reaction of phosphorus and methanol existing in metallic silicon used for the reaction with methanol.

The inventors of the present invention surprisingly reduced the content of trimethoxymethylsilane in the process of examining the separation of trimethoxymethylsilane and tetramethoxysilane and the study of the separation of high-boiling components, and surprisingly It was also found that the phosphorus content decreases as the temperature difference between the top and bottom of the distillation is reduced. That is, according to the production method of the present invention, tetramethoxysilane having a phosphorus content of 10 ppb or less can be easily obtained by keeping the temperature difference between the tower top and the tower bottom at 20 ° C. or less during the separation distillation of high boiling components. Of. In addition,
The content of trimethoxymethylsilane may be measured by a known measuring method, and for example, by using gas chromatography such as FID method, it can be measured with high sensitivity. The method for measuring the phosphorus content is not particularly limited, and various known methods can be used as long as they have sufficient sensitivity. For example, tetramethoxysilane is hydrolyzed with ultrapure water to form SiO 2. It can be accurately detected by means such as dissolution in hydrofluoric acid as _{2 and} measurement by ICP-MS method.

The high-purity tetramethoxysilane of the present invention preferably has a small content of components other than trimethoxymethylsilane and phosphorus. For example, the residual amount of unreacted methanol is preferably 0.3% by weight or less, but these other components are usually removed at the same time in the process of removing trimethoxymethylsilane. In any case, the phosphorus content control can be achieved by the production method of the present invention.

When the synthetic quartz powder is produced by the sol-gel method using the high-purity tetramethoxysilane thus obtained, the quartz glass produced by using the synthetic quartz powder does not substantially contain phosphorus. The molded body also contains substantially no phosphorus. Such a quartz glass molded body is extremely useful, for example, even when used as a crucible for pulling up a silicon single crystal, since phosphorus is not eluted from the crucible, a silicon single crystal containing substantially no phosphorus can be obtained. is there.

[0029]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples. [Reference Example-1] Using dodecylbenzene as a solvent, in the presence of a copper catalyst, metal silicon powder and methanol were reacted to obtain a mixed composition of tetramethoxysilane, trimethoxymethylsilane, unreacted methanol and dodecylbenzene, Substantially the entire amount of trimethoxymethylsilane was converted to tetramethoxysilane using calcium carbonate as a catalyst to obtain the compositions shown in Table 1.

[0030]

[Table 1] HB: Silicon-containing oligomer and insoluble matter such as calcium carbonate

Subsequently, the compositions shown in Table 1 were continuously subjected to a thin film evaporator to separate HB and dodecylbenzene at 110 ° C. to obtain the compositions shown in Table 2.

[0032]

[Table 2]

[Example-1] Using a glass Oldershaw type distillation apparatus having a reflux condenser at the top of the column (the number of stages: 20), at atmospheric pressure and in a batch operation (reflux ratio: 3), the table of the reference example-
The composition described in 2 was distilled. After collecting the effluent containing methanol as a main component with a column top temperature of less than 120 ° C as the first fraction, the fraction with a column top temperature of 120 to 121 ° C is collected as the main fraction until the reboiler temperature rises to 126 ° C. did.
The maximum temperature difference between the tower top and the tower bottom was 5 ° C. When this distillate was analyzed, it had the composition shown in Table 3. Moreover, the phosphorus content was measured and found to be 0.8 ppb (the phosphorus content is a value measured by the ICP-MS method).

[0034]

[Table 3]

[Example-2] The column top temperature at the time of main distillation collection was set to 8
Distillation was performed in the same manner as in Example-1, except that the temperature was set to 0 to 121 ° C. When the collected composition was analyzed, Table-4
The composition shown in FIG. The phosphorus content was measured and found to be 4 ppb.

[0036]

[Table 4]

[Examples 3, 4 and Comparative Example 1] Using a packed column distillation apparatus made of stainless steel (theoretical plate number: 14 plates) having a reflux condenser at the top of the column, atmospheric pressure and continuous operation (reflux ratio: 3) )
Then, after distilling the composition shown in Table 2 of Reference Example to separate low boiling components, a packed tower type distillation apparatus made of stainless steel (theoretical plate number: 14
The high boiling components were separated by distillation under normal pressure and continuous operation.
The distillation conditions for the low-boiling components were the same, and the operation was performed by changing only the distillation conditions for the high-boiling components. That is, the temperatures at the top and bottom of the column during the distillation separation of the high boiling components were controlled to the values shown in Table-5. When the collected composition was analyzed, the composition was as shown in Table-5. Also, the phosphorus content was measured and found to be Table-5.
It was the value shown in.

[0038]

[Table 5]

[Examples-5, 6 and Comparative Example-2] The tetramethoxysilanes obtained in Examples-3, 4 and Comparative Example-1 were each reacted with water to obtain a lumpy wet gel. It was After crushing to a particle size of 1 mm or less, it was dried to obtain a powdery dry gel. Subsequently, the powdery dry gel was placed in a quartz container and kept in an electric furnace at 1200 ° C. for 30 hours. The phosphorus content in the obtained quartz powder,
The values are shown in Table-6.

[0040]

[Table 6]

[Examples-7, 8 and Comparative Example-3] The quartz powders obtained in Examples-5, 6 and Comparative Example-2 were melted (Bernoulli method) in an acid / hydrogen flame to obtain a rod-shaped ingot. It was created. The phosphorus contents present in the ingot were the values shown in Table-7, respectively.

[0042]

[Table 7]

[0043]

Industrial Applicability According to the present invention, high-purity tetramethoxysilane useful as a raw material for synthetic quartz powder or a synthetic quartz compact is obtained.

Claims (2)

[Claims] 1. A method for purifying crude tetramethoxysilane obtained by the reaction of metallic silicon and methanol to produce high-purity tetramethoxysilane, wherein a high boiling point component is separated from crude tetramethoxysilane at the top of the column. And a column bottom temperature difference of 20 ° C. or less for precision distillation to produce a high-purity tetramethoxysilane. 2. Distillation purification is carried out so that the content of trimethoxymethylsilane in tetramethoxysilane after separating the low boiling point component from crude tetramethoxysilane is 0.3% by weight or less. 1. The method for producing high-purity tetramethoxysilane according to 1.