JPH11349593A - Production of high purity tetraalkoxysilane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industriously advantageously obtain a high purity tertraalkoxysilane useful as a raw material for synthetic quartz glass powder or the like by reacting silicon with an alcohol and by recovering the unreacted alcohol to bring it into contact with activated carbon and to recycle it to the reaction of it with silicon. SOLUTION: When producing a tetraalkoxysilane by reacting silicon with an alcohol, the method of this invention is to recover part or the whole of unreacted alcohol, to bring it into contact with activated carbon and to recycle it to the reaction of it with silicon. The method gives a high purity tetraalkoxysilane industriously advantageously. The high purity tetraalkoxysilane gives synthetic quartz glass powder through sol-gel processing, which contains phosphorus in <=200 ppb content and is useful as raw materials or the like for synthetic quartz glass molded products used in the field of optical communication and in the semiconductor industry.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-purity tetraalkoxysilane useful as a raw material for synthetic quartz glass powder and a quartz glass molded body. 2. Description of the Related Art In recent years, in glass products used in the optical communication field, the semiconductor industry, and the like, very strict control has been performed on trace impurities. For example, when the glass product is a silicon single crystal pulling crucible and contains trace impurities, the trace impurities eluted from the crucible are mixed into the silicon single crystal and deteriorate the performance of the semiconductor. In particular, the group III and V elements, such as phosphorus and boron, affect the control of semiconductor characteristics, and therefore, great care is taken. Further, when used as a quartz jig used at a high temperature, Group III and V elements contained in the quartz glass compact diffuse and contaminate the silicon wafer, so that desired semiconductor characteristics cannot be obtained.

The high-purity quartz glass used for such applications is mainly composed of (1) a method for purifying natural quartz, and (2) fumes generated by decomposition of silicon tetrachloride in an oxyhydrogen flame. It is produced by a method of attaching and growing, (3) a method of using a synthetic quartz powder obtained by firing a gel obtained by hydrolysis and gelation of silicon alkoxide and the like, and the like.

[0003]

However, the method (1) has a limit in reducing the content of trace impurities, and the method (2) has a problem that the production cost is extremely high. In addition, although the method (3) produces a synthetic quartz glass powder having a relatively low impurity content, the content of boron and phosphorus has not always been reduced to a satisfactory level.

On the other hand, when purifying crude tetramethoxysilane obtained by the reaction between metallic silicon and methanol to produce high-purity tetramethoxysilane, the overhead of separating a high-boiling component from crude tetramethoxysilane is high. A method of separating boron and phosphorus existing in the form of a high boiling compound by precision distillation with the temperature difference between the water and the bottom at 20 ° C. or less (Japanese Patent Application Laid-Open No. 9-143184). When synthesizing alkoxysilane, purging at least 2% of unreacted alcohol recovered from the reaction product suppresses accumulation of low-boiling impurities having a relatively close boiling point to alcohol, thereby obtaining a high-purity alkoxysilane ( Japanese Patent Application Laid-Open No. 9-95491) has been proposed. However, in this case,
If an industrially inexpensive material with a high impurity content is used as the metal silicon raw material, it is necessary to increase the purge amount in order to suppress the accumulation of low-boiling impurities in the recovered alcohol to a low level without any problem, which increases the production cost. Leads to. In addition, it has been clarified by the present inventors that phosphorus or boron particularly forms a low-boiling compound in this reaction system and adversely affects the quality of quartz glass products.

[0005]

Means for Solving the Problems In view of the above-mentioned circumstances, the present inventors have made intensive studies on a method for producing tetraalkoxysilane having a lower phosphorus and boron content than conventional ones. As a result, by subjecting the unreacted alcohol recovered from the reaction product to a specific treatment, phosphorus or boron present as a low-boiling compound, which is a problem, is changed to a high-boiling compound to form a crude tetraalkoxysilane. Can be easily separated in the distillation step of
The present inventors have found that it is possible to produce tetraalkoxysilane, which has a very low boron content and can be suitably used for a quartz glass member related to semiconductor production, and has reached the present invention. That is, the present invention, in the production of tetraalkoxysilane to react silicon and alcohol, recover a part or all of the unreacted alcohol, and contact with activated carbon,
The present invention relates to a method for producing tetraalkoxysilane, which is characterized in that it is recycled for reaction with silicon.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The tetraalkoxysilane in the present invention has the following (1)
As shown in the formula, it is obtained by reacting metal silicon with an alcohol. Si + 4ROH → Si (OR) 4 + 2H 2 (1): the amount of (R group) alcohol, to silicon 1 mole, 3-50
The molar ratio is particularly preferably selected from the range of 4 to 10 moles. If the molar ratio is less than 3 times, a large amount of unreacted silicon remains and productivity is low. If the molar ratio exceeds 50 times, a large amount of unreacted alcohol remains and a large amount of energy is required for separation and purification.

As the reaction solvent, a hydrocarbon solvent can be widely used, but a high boiling point solvent which is easily separated from tetraalkoxysilane is particularly suitable for use. Examples of the high boiling point solvent include an alkylbenzene compound such as triethylbenzene, octylbenzene, dodecylbenzene, didodecylbenzene and the like, and an arylmethane compound such as diphenylmethane, benzyltoluene and dibenzyltoluene. Of these, dodecylbenzene is particularly preferred because it has a large boiling point difference from the target tetraalkoxysilane, is easily separated, and is relatively inexpensive. The amount of solvent used is industrially 1 g of metallic silicon.
In contrast, 1 ml to 100 ml, especially
It is better to select appropriately from the range of 1.5 to 10 ml. If the amount of the solvent is too small, the metal silicon cannot be turned into a slurry state, so that the reaction cannot be performed. If the amount of the solvent is too large, productivity per unit is low and a large-sized apparatus is required.

As a catalyst for this reaction, a base catalyst such as an alkali alcoholate (JP-A-52-12133)
And copper catalyst such as copper chloride (JP-B-50-34538)
It has been known. The reaction temperature varies depending on the solvent, the catalyst and the like, but is usually 100 to 300 ° C. The pressure during the reaction is a value obtained by adding the pressure of the hydrogen gas generated by the reaction to the vapor pressure of the alcohol and the hydrocarbon at the above reaction temperature. A reaction can also be performed.

In addition to the above one-step reaction, once a mixed composition of tetraalkoxysilane, trialkoxyhydrosilane, and alcohol is obtained, an alkali metal or alkaline earth metal oxide or hydroxide is used as a catalyst, A method for obtaining tetraalkoxysilane has also been proposed (JP-A-63-166888). The reaction composition obtained as described above (hereinafter, “crude tetraalkoxysilane”)
) Contains a solvent, an unreacted alcohol, an insoluble component, a trialkoxyalkylsilane as a by-product, and a silicon-containing oligomer in addition to the target substance, tetraalkoxysilane.

[0010] By purifying the crude tetraalkoxysilane, a high-purity tetraalkoxysilane is obtained.
In performing efficient purification, for example, the following methods are mentioned. First, insoluble components are separated. Insoluble components are
It is dispersed as a solid in the reaction solution, and can be separated by filtration, or can be separated as a distillation residue together with a part of high boiling components using a thin film evaporator. Insoluble components in the crude tetraalkoxysilane often have poor filterability, so that the latter method is preferred for industrial use.

In the purification method after removing the insoluble components, first, components having a lower boiling point than tetraalkoxysilane are separated by distillation. The low-boiling component referred to here is a component having a lower boiling point than tetraalkoxysilane, such as unreacted alcohol and trialkoxyalkylsilane. Distillation separation
Conventional distillation equipment can be used. The operation can be performed either batchwise or continuously. To accurately separate the tetraalkoxysilane from the alcohol and the trialkoxyalkylsilane, a tray or a packing is provided in the distillation column. The number of stages is usually 5 to 50, and the column diameter is determined by the processing conditions. The pressure is usually operated at 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 to the column, and the reflux ratio is usually 0.1 to 5. In the case of performing distillation in batch, the operation may be performed while changing the reflux ratio according to the composition of the liquid to be distilled. In batch operation, following low boiling components,
High-purity tetraalkoxysilane will be distilled out, but in a continuous operation, tetraalkoxysilane will be extracted from the reboiler.

[0012] The content of trialkoxyalkylsilane, which is a by-product of the reaction, in the tetramethoxysilane after separating low boiling components from the crude tetramethoxysilane is 0.3%.
It is preferable to purify by distillation so as to be not more than% by weight.
For example, in the case of tetramethoxysilane, the structure of a phosphorus compound having a lower boiling point than the above was found by analysis of the present inventors to be mainly trimethylphosphine P (CH ₃ ) _3. Experiments conducted by the present inventors have confirmed that when distillation is performed under conditions where the content of trimethoxymethylsilane in methoxysilane exceeds 0.3% by weight, the concentration of phosphorus sharply increases.

On the other hand, the low-boiling components recovered under the above conditions are mostly unreacted alcohols, and industrially, it is preferable to recycle this alcohol and use it again for the reaction with silicon. However, in this low boiling component,
In addition to alcohols, trialkoxyalkylsilanes and low-boiling compounds of phosphorus and boron are contained, and these are accumulated by repeating recycling. And
If the accumulated amount exceeds a certain concentration, even if low-boiling components are separated under the aforementioned distillation conditions, a very small amount will be mixed into the tetraalkoxysilane on the reboiler side. In particular, the allowable concentrations of phosphorus and boron in quartz glass used for semiconductor production are extremely low (a level of several tens of ppb or less depending on the use), and even a very small amount of this causes a problem.

To cope with this, as described above, in order to suppress the accumulation of these impurities as low-boiling components to a certain level or less, there is a method of purging a part of the recovered alcohol instead of recycling the entire amount (Japanese Patent Laid-Open No. 9-1997). No. 955491). However, in the case of using this method, in order to use a less expensive metal silicon having a high impurity level as a reaction raw material, it is necessary to increase the purge amount of the recovered alcohol, which is disadvantageous industrially and economically. Is as stated.

On the other hand, the present inventors surprisingly performed a simple operation of bringing the recovered alcohol into contact with the activated carbon, and surprisingly, without purging the alcohol, the phosphorus in the synthetic quartz glass finally obtained was obtained. And boron can be reduced. In the present invention, the mechanism of reducing phosphorus and boron by contact with activated carbon is not clear, but for example, when synthesizing tetramethoxysilane, the main component of the low-boiling compound present in the recovered methanol, P (C
For H ₃ ) ₃ (trimethylphosphine: boiling point 38 ° C.), the morphological change after activated carbon treatment was identified by gas chromatography analysis and NMR (nuclear magnetic resonance) analysis.
(CH3) ₃ P = O (trimethylphosphine oxide: boiling point: 215 ° C.). This is presumably because the oxidation reaction of the activated carbon by the oxidation functional group or the adsorbed oxygen changed the low-boiling P compound into a higher-boiling oxide. As described above, it is considered that a low-boiling phosphorus compound can be changed to a compound having a considerably higher boiling point than tetramethoxysilane (boiling point: 121 ° C.) even without the effect of adsorption and removal by activated carbon. It can be easily separated from tetramethoxysilane.

The contact time between the recovered alcohol and the activated carbon is 0.5 minutes or more, preferably 1 minute or more. In addition, the life of activated carbon cannot be determined unconditionally because the amount of adsorbed oxygen and the amount of oxidized functional group vary depending on the type of activated carbon. The contact treatment with activated carbon may be batch or continuous, but industrially, a method of passing recovered alcohol through a column filled with activated carbon is conceivable.

Next, with respect to the tetraalkoxysilane from which the low boiling components have been removed, the high boiling components are further separated by distillation.
The high-boiling component referred to here is a silicon-containing oligomer, a solvent,
It is a component having a higher boiling point than tetraalkoxysilane, such as an oxide compound of phosphorus or boron. For 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 in a batch operation, high-purity tetraalkoxysilane is distilled off following the low-boiling component, and can be separated from the high-boiling component by distillation using the same distillation apparatus. On the other hand, when performing continuous distillation of low-boiling components, tetraalkoxysilane is extracted from the reboiler, and the extracted liquid is supplied to a distillation apparatus installed in series to separate high-boiling components by distillation. I can do it.

In order to accurately separate the tetraalkoxysilane from the silicon-containing oligomer, the solvent and the like, a tray or a packing is provided in the distillation column. The number of stages is 5
At ~ 50 stages, the tower diameter is determined by the processing conditions. The pressure is usually operated at 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, and the reflux ratio is usually 0.1 to 5. In the case of performing distillation in batch, the operation may be performed while changing the reflux ratio according to the composition of the liquid to be distilled.

Further, when separating high boiling components from the crude tetraalkoxysilane, the temperature difference between the top and bottom of the column is 20 ° C.
It is preferable to purify by distillation as follows. For phosphorus compounds having a higher boiling point than tetraalkoxysilane, it is not clear other than trialkylphosphine oxide, which is the main component, but when distilled and purified by setting the temperature difference between the top and bottom to 20 ° C. or less, the concentration becomes 10 ppb or less. Experiments have shown that we can do it. Even when the temperature difference between the top and the bottom exceeds 20 ° C., the silicon-containing oligomer and the solvent can usually be substantially completely separated, but the temperature difference is 2
Above 0 ° 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 stopped when the temperature difference between the top and bottom of the column reaches 20 ° C. On the other hand, when the operation is performed by the continuous method, the temperature difference between the top and bottom of the column is controlled by controlling the concentration of tetraalkoxysilane in the reboiler solution.
It can be kept below ℃. When the purification after removing the insoluble components is performed by a continuous method, first, a high-boiling component can be separated from tetraalkoxysilane, and then a low-boiling component can be separated. Since pure tetraalkoxysilane is finally extracted from the reboiler and a trace metal is mixed in from the apparatus at the time of operation, it is preferable to perform distillation treatment again to achieve high purity.

Using the thus obtained tetraalkoxysilane as a raw material, a synthetic quartz glass powder can be produced by a sol-gel method. As a result of various studies, phosphorus and boron in the tetraalkoxysilane have been found. An extremely good positive correlation is found between the content and the synthetic quartz glass powder, and also the phosphorus content in the quartz glass compact obtained by melt-molding this synthetic quartz glass powder. Although the form of phosphorus in the tetraalkoxysilane is not clear except for a part, it is considered that any of them is an organic compound formed by a reaction between phosphorus and boron present in the metal silicon used in the reaction and an alcohol.

By using the method of the present invention, tetraalkoxysilanes having extremely low phosphorus and boron contents can be stably obtained in a system for industrially recovering and reusing unreacted alcohol, with almost no alcohol purge. When a synthetic quartz powder is produced by the sol-gel method using the tetraalkoxysilane obtained as described above, the synthetic quartz powder contains substantially no phosphorus or boron (several ppb level or less). The quartz glass molded body manufactured using the powder is substantially free of phosphorus and boron.
When such a quartz glass molded article is used, for example, as a crucible for pulling a silicon single crystal, phosphorus and boron are hardly eluted from the crucible, so that a silicon single crystal substantially free of phosphorus and boron can be obtained. It is extremely useful.

The production of the synthetic quartz glass powder or quartz glass compact by the sol-gel method in the present invention can be carried out by a known sol-gel method except that the tetraalkoxysilane obtained by the present invention is used as a raw material. That is, the method includes a gelation step of hydrolyzing tetraalkoxysilane as a raw material and further proceeding a condensation reaction to obtain a gel, and a step of vitrifying the obtained gel by heat treatment. First, the hydrolysis of tetraalkoxysilane in the sol-gel method is performed by reacting tetraalkoxysilane with water (generally, 1 to 10 equivalents of the alkoxy group of tetraalkoxysilane) by a known method. The obtained gel is subdivided and then dried, or dried and subdivided to obtain a gel powder, which is further heated and vitrified to obtain a synthetic quartz glass powder.

In this way, the synthetic quartz glass powder of the present invention can be obtained, but this can suppress the phosphorus content to an extremely low level. For example, 20ppb
In the following, it is even possible to set it to 10 ppb or less. Here, the method for measuring the phosphorus content is not particularly limited, and various known methods can be used as long as the method has sufficient sensitivity.For example, tetraalkoxysilane can be prepared using ultrapure water. It can be hydrolyzed to dissolve in hydrofluoric acid as SiO ₂ , and can be accurately detected by means such as measurement by an ICP-MS method.

Further, the synthetic quartz glass powder of the present invention can be further melt-molded to form a quartz glass molded body. The molding method at this time includes an arc melt method, a Bernoulli method,
What is necessary is just to select suitably according to the use of a molded object, such as a fusion method. Of course, this does not preclude directly forming a molded article by the sol-gel method using the high-purity tetraalkoxysilane obtained by the present invention.

[0026]

EXAMPLES The present invention will be described more specifically with reference to the following examples. The phosphorus concentration was measured by ICP-MS analysis, and the concentrations of tetramethoxysilane, trimethoxysilane, unreacted methanol and dodecylbenzene were determined by gas chromatography using the FID method. Reference Example 1 Metallic silicon powder was reacted with methanol in the presence of a copper catalyst using dodecylbenzene as a solvent to obtain a mixed composition of tetramethoxysilane, trimethoxysilane, unreacted methanol and dodecylbenzene, and then oxidized. Substantially all of trimethoxysilane was converted to tetramethoxysilane using calcium as a catalyst to obtain the compositions shown in Table 1.

[0027]

[Table 1] HB: Insolubles such as silicon-containing oligomer and calcium oxide Subsequently, the compositions shown in Table 1 were continuously supplied to a thin film evaporator to separate HB and dodecylbenzene at 110 ° C.
The compositions described in Table 2 were obtained.

[0028]

[Table 2] (Example 1) Using a glass-made Oldershaw-type distillation apparatus having a reflux condenser at the top of the column (number of stages: 20), normal pressure and batch operation (reflux ratio: 3), as shown in Table-2 of Reference Example The composition was distilled. After recovering a distillate mainly composed of methanol having an ascending temperature of less than 120 ° C. as an initial distillate a (hereinafter referred to as unreacted methanol), the tower top temperature is from 120 to 1
A fraction at 21 ° C. was collected as main fraction a until the temperature of the reboiler rose to 126 ° C. At this time, the maximum temperature difference between the top and the bottom was 5 ° C. Analysis of the obtained unreacted first-reacted methanol and the main distillate showed the compositions shown in Tables 3 and 4 (in terms of the measurement of phosphorus, IC
P-MS analysis).

[0029]

[Table 3]

[0030]

[Table 4] * Phosphorus concentration unit: ppb Next, the recovered unreacted unreacted methanol is used again as a raw material in Reference Example 1 to carry out a reaction, and the same distillation conditions are used to separate the first distillation and the main distillation. went. The operation of repeatedly recycling the unreacted methanol of the first distillation into a reaction raw material with metallic silicon was repeatedly performed for 10 cycles. Tables 5 and 6 show the results of the composition analysis of the first distillation b and the main distillation b obtained by the distillation operation in the tenth cycle.

[0031]

[Table 5]

[0032]

[Table 6] As described above, by repeating the recycle use of unreacted methanol, accumulation of trimethoxymethylsilane and phosphorus was recognized in the first distillation methanol, and particularly, the concentration of phosphorus in the main distillation significantly increased. Through the steps of hydrolysis, gelation, firing, etc. of tetramethoxysilane, when synthetic quartz glass is used, since it is concentrated about 2.5 times,
On a SiO ₂ basis, this corresponds to a phosphorus concentration of just over 20 ppb.

Therefore, for unreacted methanol having the composition shown in Table 5, a glass cylindrical vertical type activated carbon packed column was used.
After passing through at a flow rate such that the contact time with activated carbon was 10 minutes, the mixture was reacted with metallic silicon, and the first distillation c and the main distillation c were collected by distillation under the same conditions as described above. The results of each composition analysis are shown in Table-7 and Table-8.

[0034]

[Table 7]

[0035]

[Table 8] From these results, it was confirmed that by performing the contact treatment of the phosphorus with the activated carbon under the above conditions, the amount of the mixture significantly reduced in both the first distillation and the main distillation.

[0036]

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

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takashi Yamaguchi 1-1 Kurosaki Castle Stone, Yawatanishi-ku, Kitakyushu City Inside the Mitsubishi Chemical Corporation Kurosaki Office

Claims (5)

[Claims] 1. A method for producing a tetraalkoxysilane in which silicon is reacted with an alcohol, wherein part or all of the unreacted alcohol is recovered, brought into contact with activated carbon, and recycled for the reaction with silicon. A method for producing tetraalkoxysilane. 2. A method for producing a synthetic quartz glass powder, wherein a sol-gel method is carried out using the tetraalkoxysilane obtained by the method according to claim 1 as a raw material. 3. A synthetic quartz glass powder obtained by the method according to claim 2. 4. The synthetic quartz glass powder according to claim 3, wherein the P content is 20 ppb or less. 5. A method for producing a synthetic quartz glass molded body, comprising melt-molding the synthetic quartz glass powder according to claim 3 or 4.