GB2174687A - Method of preparing partially fluorinated silane from partially chlorinated silane - Google Patents

Abstract

Preparation of a partially fluorinated silane, SiHnF4 n (n is from 1 to 3), by reaction of a partially chlorinated silane with a metal fluoride such as SbF3. The reaction is carried out by first mixing the metal fluoride with an organic liquid so as to provide a reaction medium, which is in a liquid state and may be a suspension, and blowing a partially chlorinated silane gas into the reaction medium.

Description

SPECIFICATION Method of preparing partially fluorinated silane from partially chlorinated silane This invention relates to a method of preparing a partially fluorinated silane from a partially chlorinated silane.

Partially fluorinated silanes represented by the general formula SiHnF4n (n is from 1 to 3) have increasing uses in the fields of electronic and optical devices. For preparation of partially fluorinated silanes, direct fluorination of silane SiH4 is rather unsuitable since it is very difficult to control the degree of fluorination. Therefore, it is customary to obtain a desired partially fluorinated silane by subjecting a partially chlorinated silane to a halogen substitution reaction using a suitable fluorinating agent. Partially chlorinated silanes can easily be prepared, for example, by reaction between silicon and chlorine or hydrogen chloride or by reaction between silane and hydrogen chloride.

For the halogen substitution reactions to convert partially chlorinated silanes to partially fluorinated silanes, a group of metal fluorides have been preferred as the fluorinating agent. Because of using a solid material as the fluorine source, it is usual to carry out the substitution reaction to form a desired partially fluorinated silane by passing a partially chlorinated silane gas through a column or tower packed with a metal fluoride in granular form. However, this method is not very advantageous from a practical point of view. The main reasons are as follows.

The halogen substitution reaction involves gradual conversion of the metal fluoride into metal chloride, which is accompanied by some changes in the physical properties of the packed column. This often becomes a cause of tight cohesion of the metal halide grains and clogging of the packed column, and such phenomena present difficulty in continuing the reaction. From another aspect, it is often that substantial amounts of excessively fluorinated silanes are formed as unwanted by-products. Furthermore, a considerable portion of fluorine contained in the metal fluoride cannot effectively be utilized.

It is an object of the present invention to provide a method of preparing a partially fluorinated silane from a partially chlorinated silane, which method is high in efficiency and also in the yield of a desired fluorosilane and can be put into practice without difficuity.

In short the above object is accomplished by using a mixture of a metal fluoride and an organic liquid for halogen substitution reactions of partially chlorinated silanes.

More definitely, the present invention provides a method of preparing a partially fluorinated silane represented by the general formula SiH"F4 n, where n is from 1 to 3, by reaction between a partially chlorinted silane gas and a metal fluoride, and this method is characterized in that the metal fluoride is mixed with an organic liquid so as to provide a reaction medium in a liquid state and that the partially chlorinated silane gas is blown into the reaction medium.

In this method the metal fluoride is selected from ones known as fluorinating agents for converting chlorosilanes to fluorosilanes, and it is preferred to use SbF3 or ZnF2. In every case dissolution of the metal fluoride in the employed organic liquid is not a requisite. There are some cases where almost complete suspension of the metal fluoride in the organic liquid is preferable and different cases where partial or almost complete dissolution of the metal fluoride is rather preferable.

An advantage of the method according to the invention is that the desired substitution of fluorine for chlorine in the starting material can be accomplished easily and almost completely while formation of undesirably highly fluorinated silanes is very or fairly little. Also it is an advantage of this method that the metal chloride formed by reaction of the metal fluoride with the partially chlorinated silane does not offer obstruction to further continuation of the reaction since the metal chloride is soluble in various organic solvents from which the liquid of the reaction medium according to the invention can be selected. Whether the metal fluoride in the reaction medium is suspending or dissolving, there is no difference in dissolution of the metal chloride formed by the reaction.Furthermore, in the method according to the invention the fluorine utilization factor becomes very high, and in many cases it is possible to effectively utilize almost the entire amount of fluorine of the employed metal fluoride in the intended halogen substitution reaction.

In the present invention the starting chlorosilane gas may be any of SiHCI3 gas, SiH2CI2 gas and SiH3CI gas. That is, any of SiHF3, SiH2F2 and Si3F can be prepared by a method according to the invention. It is also possible to use a mixture of two or three kinds of partially chlorinated silanes. However, when it is wished to obtain a single kind of partially fluorinated silane it is favorable to perform an isolation operation at the stage of partially chlorinated silanes since chlorosilanes are more different in their boiling points than fluorosilanes.

As mentioned hereinbefore, it is preferred to use SbF3 or ZnF2 as the fluorinating agent. SnF4 too is a suitable metal fluoride. It is possible, if desired, to use a still different metal fluoride such as TiF4, CrF3 or VF3. It is most favorable to use SbF3 because this fluoride is superior in reactivity. Accordingly, when SbF3 is used an intended halogen substitution reaction can smoothly be carried out even at relatively low temperatures.

In selecting an organic liquid to be mixed with a metal fluoride, consideration should be given to the intended reaction temperature and also to the solubility of the metal fluoride in the selected organic liquid at the reaction temperature. It will be understood that the reaction temperature should be fairly lower than the boiling point of the organic liquid.

When SbF3 is used it is favorable for enhancement of the yield of a desired partially fluorinated silane to select an organic liquid in which SbFs is hardly soluble. This is because dissolution of a substantial amount of SbF3 in the employed organic liquid is liable to cause an increase in the formation of undesirably highly fluorinated silanes. More particularly, it is preferable to use an organic liquid in which the solubility of SbF3 at the reaction temperature is not more than 1 g/l. Such a liquid can be selected from, for example, heptane, hexane, n-butyl ether, benzene, toluene, chlorobenzene and nitrobenzene.It is favorable to carry out the reaction between a partially chlorinated silane gas and SbF3 at a temperature as low as possible and practical because in a given solvent the solubility of SbF3 becomes lower, and the yield of the desired fluorosilane becomes higher, as the temperature is lowered, and also because evaporation loss of the solvent becomes smaller at low temperatures. Accordingly the reaction of medium prepared by using SbF3 becomes a suspension. To accomplish uniform reaction, it is desirable to continue efficient stirring of this suspension while a partially chlorinated silane gas is blown into the suspension.When SbF3 is used, a suitable range of the reaction temperature is from about -500C to about 50 C. Of course an optimum reaction temperature depends on the melting point and the boiling point of the employed organic liquid.

In the case of using ZnF2 or any other kind of metal fluoride which is considerably lower in reactivity than SbF3, it is suitable to carry out the reaction at a relatively high temperature, which is usually in the range of from about 50 C to about 100"C. In combination with such a metal fluoride, it is desirable to use an organic solvent in which the metal halide is substantially soluble. By using such a solvent the rate of the desired halogen substitution reaction is enhanced, whereas the probability of fluorination of silane to undesirably high degrees is lessened.

For example, it is preferable to use n-butyl ether or nitrobenzene in combination with ZnF2.

In the reaction medium according to the invention the proportion of the metal fluoride to the organic liquid does not need to be strictly limited. In general, it is suitable that the metal fluoride amounts to 0.1-50 wt% of the organic liquid.

The invention will further be illustrated by the following nonlimitative examples.

EXAMPLE 1 The fluorinating agent was antimony trifluoride which was refined by sublimation and was sieved to use only a portion that passed through 60-mesh and remained on 100-mesh. First, 90 g of SbF3 and 300 ml of completely dehydrated heptane were charged in a round-bottom flask having a capacity of 500 ml. A mercury-sealed stirier was provided to the flask, and the air in the flask was completely replaced by helium. After that the suspension of SbF3 in heptane was kept cooled at about 0 C, and, continuing stirring, a dichlorosilane gas was blown into the suspension at a constant rate of 160 ml/min. The dichlorosilane gas was comprised of 99.0% of SiH2CI2, 0.5% of SiHCI3, 0.4% of HCI and 0.1% of SiCI4. The gas emerged from the suspension was recovered and was analyzed by gas chromatography at intervals.

After 60 min from the start of the reaction, the composition of the fluorinated gas was as shown in Table 1. The feed of the dichlorosilane gas was further continued until the amount of unreacted SiH2CI2 in the fluorinated gas reached 3%. The total duration of the reaction was 102 min. For this duration, the utilization factor of fluorine, i.e. ratio of the quantity of SbF3 consumed in the reaction to the quantity of initially charged SbF3, was 97.2%.

EXAMPLES 2-7 Examples 2-7 were repetitions of the fluorination process in Example 1 except that different solvents were used in place of heptane in Example 1, as shown in Table 1, and that the reaction temperature was raised in Examples 6 and 7 as shown in Table 1.

In Examples 2-7, the results of analysis of the fluorinated gas after 60 min from the start of the reaction were as shown in Table 1. In every example the reaction was continued until finding of 3% of unreacted SiH2CI2 in the fluorinated gas, so that the duration of reaction was variable as shown in Table 2.

TABLE 1 Organic Reac- Solubil- Composition of Obtained Gas (vol%) Solvent tion ity of Temp. SbF3 SiH2F2 SiHF3 SiF4 HCl H2 (OC) (g/l) Ex. 1 heptane 0 0.016 98.5 0.8 0.2 0.5 Ex. 2 hexane 0 0.020 98.6 0.7 0.2 0.5 Ex. 3 toluene 0 0.005 98.9 0.6 0.1 0.4 Ex. 4 n-butyl 0 0.5 74.3 7.1 7.0 1.1 10.5 ether Ex. S chioro- benzene 0-005 98.9 0.6 0.1 0.4 - Ex. 6 benzene 10 0.0055 98.9 0.6 0.1 0.4 nitro- Ex. 7 nitro 25 0.56 69.0 7.4 9.3 1.3 13.0 TABLE 2 Duration of Fluorine Utilization Reaction (min) Factor (8) Ex. 1 102 97.2 Ex. 2 103 97.5 Ex. 3 104 98.9 Ex. 4 92 98.2 Ex. 5 103 97.2 Ex. 6 102 96.6 Ex. 7 87 98.7 EXAMPLE 8 A monochlorosilane gas was used as the starting material and toluene as the organic solvent in which SbF3 was dispersed. Otherwise, the fluorination process of Example 1 was repeated.

The starting gas was comprised of 99.0% of SiH3CI and 1.0% of SiH2Cl2.

After 60 min from the start of the reaction the composition of the fluorinated gas was: 98.2% SiH3F, 1.2% SiH2F2, 0.3% SiHF3, 0.1% SiF4 and 0.2% HCI by volume.

EXAMPLE 9 A trichlorosilane gas was used as the starting material and toluene as the organic solvent.

Otherwise, the fluorination process of Example 1 was repeated. The starting gas was comprised of 99.8% of SiHCI3 and 0.2% of SiCI4.

After 60 min from the start of the reaction the composition of the fluorinated gas was: 99.5% SiHF3, 0.3% SiF4 and 0.2% HCI by volume.

EXAMPLE 10 The dichlorosilane gas used in Example 1 was fluorinated by the same method as in Example 1 except modifications in the following points.

ZnF2 was used as the fluorinating agent and n-butyl ether as the organic solvent. The reaction was carried out at 600C.

After 60 min from the start of the reaction the composition of the fluorinated gas was: 98.3% SiH2F2, 1.0% SiHF3, 0.4% SiF4 and 0.3% HCI by volume.

EXAMPLE 11 The dichlorosilane gas used in Example 1 was fluorinated by the same method as in Example 1 except modifications in the following points.

1,4-Dioxane was used as the organic solvent in which SbF3 was dispersed. The reaction was carried out at 25"C.

After 60 min from the start of the reaction the composition of the fluorinated gas was: 60.9% SiH2F2, 8.8% SiHF3, 11.0% Six4, 3.9% HCI and 15.4% H2 by volume.

COMPARATIVE EXAMPLE 90 g of granular antimony trifluoride, which was refined by sublimation and had a mean grain size of 2 mm, was packed in a laboratory tower. After replacing the air in the packed tower by helium, the temperature in the packed tower was kept at about 60"C. The dichlorosilane gas used in Example 1 was passed through the packed tower at a constant rate of 160 m!/min.

After 30 min from the start of the reaction, analysis of the fluorinated gas was: 60.9% SiH2F2, 8.8% SiHF3, 11.0% SiF4, 3.9% HCI and 15.4% H2 by volume. After 52 min from the start of the reaction the amount of unreacted SiH2CI2 in the fluorinated gas reached 3%. For this duration the fluorine utilization factor was 58.2%.

Claims (8)

1. A method of preparing a partially fluorinated silane represented by the general formula SiHnF4 n, where n is from 1 to 3, the method comprising the steps of mixing a metal fluoride with an organic liquid so as to provide a reaction medium in a liquid state and blowing a partially chlorinated silane gas into said reaction medium.

2. A method according to Claim 1, wherein said metal fluoride is selected from SbF3, ZnF2 and SnF4.

3. A method according to Claim 1, wherein said metal fluoride is SbF3, said organic liquid being selected from heptane, hexane, n-butyl ether, benzene, toluene, chlorobenzene and nitrobenzene.

4. A method according to Claim 3, wherein said reaction medium is maintained at a temperature in the range from --50"C to 500C.

5. A method according to Claim 1, wherein said metal fluoride is ZnF2, said organic liquid being selected from n-butyl ether and nitrobenzene.

6. A method according to Claim 5, wherein said reaction medium is maintained at a temperature in the range from 50"C to 1O00C.

7. A method according to any one of the preceding claims, wherein said metal fluoride amounts to 0.1-5.0 wt% of said organic liquid.

8. A method of preparing a partially fluorinated silane, substantially as hereinbefore described in any of Examples 1 to 11.