JPH0931080A - High-purity tetramethoxysilane and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high-purity tetramethoxysilane useful to synthetic quartz powder or a raw material for its compact by reacting metal silicon with methanol. SOLUTION: This compound having <=0.3wt.% trimethoxymethylsilane content, preferably <=50ppb boron content and <=10ppb phosphorus content is obtained by reacting (A) silicon metal with (B) methanol. An insoluble component, a high-boiling component and a low-boiling component are separated from crude tetramethoxysilane obtained by the reaction of the component A and the component B. The resultant substance is distilled and purified so as to make <=0.3wt.% content of trimethoxymethylsilane in tetramethoxysilane. The amount of the composition B, for example, is preferably 4-10mols based on 1mol of the component A. Dodecylbenzene, etc., are used as a solvent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-purity tetramethoxysilane useful as a raw material for synthetic quartz powder or a molded product thereof and a method for producing the same.

In recent years, in glass products used in the fields of optical communication, semiconductor industry, etc., very strict control has been carried out regarding trace impurities and micro bubbles in the products. For example, when the glass product is a crucible for pulling 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. Further, if the micro bubbles are contained, not only causes the liquid level fluctuation at the time of pulling the silicon single crystal and the occurrence of crystal defects, but also causes the problems such as the durability of the crucible to be reduced.

Such high quality glass is mainly produced by a method for purifying natural quartz, a method for adhering and growing fumes generated by decomposition of silicon tetrachloride in an oxyhydrogen flame on 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. On the other hand, when silica gel, particularly silica gel derived from silicon alkoxide, is used, a synthetic quartz powder having a relatively low content of trace impurities can be obtained, but it cannot be said that the required level is necessarily satisfied. Further, in this method, fine bubbles are easily generated in the molded product of the final product, which leads to the above-mentioned problems due to the fine bubbles.

[0005]

SUMMARY OF THE INVENTION In view of the above situation, the inventors of the present invention use silica gel obtained by hydrolysis and gelation of silicon alkoxide, and have a low content of trace impurities as compared with conventional ones. Further, as a result of earnest research on a method for producing a synthetic quartz powder or a molded product thereof in which generation of micro bubbles is extremely small, the present inventors have completed the present invention by finding out that the main cause of these problems is the specific compound in the raw material silicon alkoxide.

[0006]

The present invention provides (1) tetramethoxysilane obtained by the reaction of metallic silicon and methanol, wherein the content of trimethoxymethylsilane is 0.3% by weight or less. In a method for producing high-purity tetramethoxysilane by purifying the characteristic high-purity tetramethoxysilane and (2) crude tetramethoxysilane obtained by the reaction of metallic silicon and methanol, a component insoluble from crude tetramethoxysilane is used. Of high-purity tetramethoxysilane characterized by distilling and purifying the content of trimethoxymethylsilane in tetramethoxysilane after separating the high-boiling component and the low-boiling component to 0.3% by weight or less. Manufacturing method.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
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, a silicon-containing oligomer having a higher boiling point than the insoluble component and tetramethoxysilane and a solvent are separated. The insoluble component is one that is dispersed as a solid in the reaction solution, and is separated by filtration, or
It can be separated as a distillation residue together with high boiling components using a thin film evaporator. In order to further reduce the content of the high boiling point component, it is desirable to perform the distillation treatment again. At this time, it was confirmed by the experiments of the present inventors that the phosphorus content can be significantly reduced by setting the temperature difference between the top and the bottom of the column to 20 ° C. or less, and the concentration can be set to 10 ppb or less. .
Next, unreacted methanol and trimethoxymethylsilane, which have a lower boiling point than tetramethoxysilane, are separated by distillation.

As a purification method after removing the insoluble component, first, unreacted methanol and trimethoxymethylsilane, which have a lower boiling point than tetramethoxysilane, are separated by distillation, and then the high boiling component is separated and distilled. You can go.

When the operation is performed by the batch method, the latter method is inevitably used. An ordinary distillation apparatus is used for distillative separation of unreacted methanol and trimethoxymethylsilane, which have a lower boiling point than tetramethoxysilane. 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, and the column 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. Further, in the batch operation, high-purity tetramethoxysilane will flow out after the low-boiling component,
In continuous operation, high-purity tetramethoxysilane is withdrawn 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.

In this way, the content of trimethoxymethylsilane in tetramethoxysilane is set to 0.3% by weight or less, preferably 0.15% by weight or less to produce the high-purity tetramethoxysilane of the present invention. It is a requirement in the method. When tetramethoxysilane having a low trimethoxymethylsilane content obtained by the present invention is used to produce synthetic quartz powder by firing silica gel obtained by the sol-gel method, black powder is generated when firing silica gel. Is significantly reduced, and 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
Although it disappears substantially by demethoxylation or oxidation, the methyl group is hard to be eliminated and the oxidation rate is slow, so that it is likely to remain as unburned carbon in the synthetic quartz powder, and black particles are generated. It is considered that this is a cause of generation of bubbles during molding using quartz powder.

On the other hand, boron and phosphorus, which are the trace impurities and which have received the most attention recently, have been considered to be contaminated from the atmosphere. However, as a result of various studies conducted by the present inventors, they were mainly used as raw materials. It was found to be mixed with certain tetramethoxysilane. That is, very good positive correlations are found between the content rates of boron and phosphorus in tetramethoxysilane and the content rates of boron and phosphorus in the synthetic quartz powder or the molded product thereof. The morphology of boron and phosphorus in tetramethoxysilane has not been clarified, but they are considered to be organic boron and organic phosphorus compounds formed by the reaction of methanol with boron and phosphorus present in the metallic silicon used for the reaction.

The present inventors have surprisingly found that, in the process of separating trimethoxymethylsilane and tetramethoxysilane, the content of boron decreases as the content of trimethoxymethylsilane decreases. It was That is, by setting the content of trimethoxymethylsilane in tetramethoxysilane to 0.3% by weight or less by the production method of the present invention, tetramethoxysilane having a boron content of 50 ppb or less can be easily obtained. . According to the production method of the present invention, when the content of trimethoxymethylsilane in tetramethoxysilane is 0.001% by weight or more, the content of boron is usually 0.01 p or less.
It becomes pb or more. The content of trimethoxymethylsilane may be measured by a known measurement method, and for example, gas chromatography such as FID method can be used to measure with high sensitivity. Further, the method for measuring the content rates of boron and phosphorus 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. Dissolved in hydrofluoric acid as SiO ₂ and use ICP-M
It can be accurately detected by means such as measurement by the S method.

The content of components other than trimethoxymethylsilane, boron, and phosphorus in the high-purity tetramethoxysilane of the present invention is naturally desirably small. 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 object of the present invention can be achieved by controlling the content of trimethoxymethylsilane.

When high-purity tetramethoxysilane thus obtained is used to produce a synthetic quartz powder by a sol-gel method, it can be made substantially free of boron and phosphorus. The quartz glass molded body produced by using the above can also be substantially free of boron and phosphorus. When such a quartz glass compact is used as, for example, a crucible for pulling up a silicon single crystal, there is no elution of boron and phosphorus from the crucible, so that a silicon single crystal containing substantially no boron and phosphorus can be obtained. , Very useful.

[0022]

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] 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, and then calcium carbonate. Was used as a catalyst to convert substantially all trimethoxymethylsilane to tetramethoxysilane to obtain the compositions shown in Table 1.

[0023]

[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.

[0025]

[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 stages), at atmospheric pressure / batch operation (reflux ratio: 3), the table of the reference example-
The composition described in 2 was distilled. An effluent containing methanol as a main component and having a column top temperature of less than 120 ° C. was collected as an initial fraction, and then a fraction having a column top temperature of 120 to 121 ° C. was collected as a main fraction. 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, when the content rate of boron was measured, it was 5 ppb or less,
The phosphorus content was measured and found to be 0.8 ppb (the boron and phosphorus contents are the values measured by the ICP-MS method).

[0027]

[Table 3]

[Example-2] The column top temperature at the time of main distillation collection was 1
Distillation was performed in the same manner as in Example-1, except that the temperature was set to 18 to 121 ° C. Analysis of the collected composition
It had the composition shown in FIG. The boron content was measured to be 10 ppb, and the phosphorus content was measured to be 0.9 ppb.

[0029]

[Table 4]

[Comparative Example-1] The column top temperature at the time of main distillation collection was 8
Distillation was performed in the same manner as in Example-1, except that the temperature was set to 0 to 121 ° C. The composition collected was analyzed and shown in Table-5.
The composition was as shown in. The boron content was measured to be 60 ppb, and the phosphorus content was measured to be 1.5 ppb.

[0031]

[Table 5]

[Examples 3 and 4 and Comparative Example-2] The tetramethoxysilanes obtained in Examples-1 and 2 and Comparative Example-1 were respectively 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 boron content and the number of black particles generated in the obtained quartz powder were the values shown in Table-6, respectively.

[0033]

[Table 6] Number of black particles generated: Number of black particles detected in 50 g of quartz powder

[Examples-5, 6 and Comparative Example-3] The quartz powders obtained in Examples-3, 4 and Comparative Example-2 were melted (Bernoulli method) in an acid / hydrogen flame to obtain a rod-shaped ingot. It was created. The results of comparison of the states of bubbles existing in the ingot are shown in Table-7.

[0035]

[Table 7]

[0036]

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

Claims (4)

[Claims] 1. A high-purity tetramethoxysilane, which is tetramethoxysilane obtained by the reaction of metallic silicon and methanol and has a trimethoxymethylsilane content of 0.3% by weight or less. 2. The high-purity tetramethoxysilane according to claim 1, wherein the content of boron is 50 ppb or less. 3. The high-purity tetramethoxysilane according to claim 1 or 2, wherein the phosphorus content is 10 ppb or less. 4. A method for purifying crude tetramethoxysilane obtained by the reaction of metallic silicon and methanol to produce high-purity tetramethoxysilane, which comprises insoluble components, high-boiling components and low-boiling components from crude tetramethoxysilane. A method for producing high-purity tetramethoxysilane, which comprises distilling and refining so that the content rate of trimethoxymethylsilane in tetramethoxysilane after separation of is 0.3 wt% or less.