DE2609767C2 - Process for the preparation of orthosilicic acid esters - Google Patents

Description

3SiF ₄ + 4ROH - (

Orthosilicic acid esters are z. B. used as Binder in precision casting, shell casting and for ceramic bodies, for the surface treatment of Glassware, as hydraulic and heat transfer fluids and for hardening silicones.

There is therefore interest in making these esters as cheaper as possible; To extract raw materials.

_{Silicon tetrafluoride (SiF 4} ) can be used as the starting silicon compound, which is produced in very large quantities as a waste product in the digestion of rock phosphates in the fertilizer industry and in glass pickling plants.

Silicic acid esters are predominantly used in technology produced by the action of silicon tetrachloride on alcohols. When converting silicon tetrafluoride However, no silicic acid esters are formed with alcohol (compare e.g. DT-PS 9 46 893, DT-AS 11 25 898), because the equilibrium on which this implementation (reaction equation 1) is based completely on the left.

SiF ₄ + 4ROH ^ Si (OR) ₄ + 4H ⁺ + 4F ~ (1)

There are some finer methods known in which the equilibrium by using other Substances is shifted to the right, so that orthosilicic acid esters are formed. These well-known However, methods are not satisfactory.

In DT-PS 9 46 893, DT-PS 10 02 748 and the US-PS 28 14 635 is the implementation of silicon tetrafluoride with alcohols or phenols in liquid Phase or described in the gas phase in the presence of ammonia. These procedures are as follows Reactions are based on: silicon tetrafluoride reacts according to reaction equation (2) with alcohol and Ammonia with formation of silicic acid ester and ammonium fluoride,

SiF ₄ I- 4ROH + 4NR ₁ - Si (OR) ₁ I 4NH ₄ F (2)

The ammonium fluoride formed reacts immediately with silicon tetrafluoride according to equation (3) Formation of ammonium aluorosilicate further,

4NU., - Si (OR) ₄
+ 2 (NH ₄ ) ^ SiF ₀

(4)

4NH ₄ F -I- 2SiF ₁ - * 2 (NH ₄ ) _a SiF "

(3)

In addition to the desired orthosilicic acid esters, ammonium hexalluorosilicate is formed. In this reaction, the theoretical yield of orthosilicic acid esters, based on the SiF _{4 used} , is only 33.3%, since two thirds of the silicon tetrafluoride used are lost as ammonium hexafluorosilicate. The practical bulge, based on the SiF _{4 used} , is even less.

According to DT-PS 9 52 263 silicon tetrafluoride with alcoholates or phenolates of the metals of the Gruppella of the Periodic Table. Alcoholates are expensive and need to be awkward to look for Metals and alcohols are produced. This method is therefore of no practical importance.

DT-PS 9 55 236 describes the reaction of SiF ₄ with alcohol in the presence of metals that are not suitable for alcoholate formation, such as zinc or iron. In this procedure, however, SiF ₄ goes in a side reaction as FeSiF ₁ . lost, as a result of which the yield is drastically reduced. Another disadvantage is the high price of the powder required! Metals. Finally, due to its content of unreacted SiF _4, the approach has a highly corrosive effect.

DT-AS 11 25 898 describes the reaction of SiF ₄ with an alcohol and a sulfide of a metal whose fluoride is stable under the reaction conditions. This process requires very long reaction times at high temperature and must be carried out with the exclusion of air in order to prevent the sulfides from being oxidized to sulfur, which would make working up the batch more difficult. The batch must also be freed from the hydrogen sulfide formed by passing nitrogen through it for hours. The recycling of the hydrogen sulfide into the process is technically complex. Because of the enormous toxicity of the hydrogen sulphide, however, care must be taken to ensure that it is disposed of carefully.

According to DT-AS 11 30 799, orthosilicates are produced by reacting silicon tetrafluoride with an alcohol in the presence of certain alkali or alkaline earth salts. The metal must form a stable fluoride with the fluorine of the SiF _4. The component A of the metal salt MA must form a volatile compound with the hydroxyl hydrogen of the alcohol. The metal fluoride formed reacts with SiF ₄ to form hexafluorosilicate. This greatly reduces the ester yield, based on the SiF _{4 used.} According to the examples, the practically achievable yields are only between 15 and 19%, based on the SiF _{4 used} . Alkali and alkaline earth carbonates, carbides, oxides and cyanates are practically insoluble in alcohols. Only suspensions in alcohols can be produced, which means that very long reaction times are required. Furthermore, it must be carried out under anhydrous conditions. However, when alkali and alkaline earth oxides and carbonates are used, water is released. The use of alkali and alkaline earth chlorides results in alcoholic solutions containing hydrogen chloride, which decompose when heated with elimination of water

be able. The described removal of the water by azeotropic distillation is limited to certain alcohols and is not so complete that partial hydrolysis of the silicon tetrafluoride can be prevented. When heated under reflux of the reaction mixture further j escape significant amounts of SiF. ₁ After all, the use of alkali or alkaline earth chlorides results in acidic, halide-containing slurries that have a highly corrosive effect. Only corrosion-resistant reactors can therefore be used to carry out the reaction in practice.

The object of the invention is to create a method for the production of Orthokicse'säureestcrn by reaction of silicon tet ^ afluorid, in which the disadvantages of the known processes can be avoided and in which the orthosilicic acid esters can be obtained in high yield with a simple procedure.

The invention is based on the knowledge that this object can be achieved by ao Silicon tetrafluoride reacts with an alcohol in the presence of certain metal salts and ammonia. The Metal salt must be soluble in the alcohol and may by introducing ammonia into the alcoholic Solution not be precipitable in any form, as Furthermore, the fluoride of this metal must be less soluble than it should be in ammoniacal alcohol Oxafluorosilicate. In addition, the metal fluoride formed must be less soluble than ammonium fluorosilicate.

Surprisingly, it has been found that in such a procedure, despite the use of Ammonia is not formed with hexafluorosilicate, but in contrast to the known with ammonia The entire SiF used can, in theory, be converted into orthosilicic acid oil can.

The invention relates to a process for the preparation of orthocic acid esters for implementation of silicon tetrafluoride with alcohols in the presence of metal salts, which is characterized is that the silicon tetrafluoride with an alcohol in the presence of lithium chloride, bromide, nitrate, sulfate or carbonate and calcium chloride, bromide or nitrate or mixtures thereof and ammonia is implemented. Calcium chloride and lithium chloride are preferred.

The reaction of the silicon tetrafluoride with the alcohol in the presence of ammonia and the metal salt (MX or MX ₂ ) takes place according to the following reaction equations (5) and (6):

SiF ₄ + 4ROH + 4MX + 4 NH.,

-> Si (OR)., I- 4MF -I- 4NH ₄ X (5) or

4NH ₄ X (6)

SiF ₄ -I- 4ROH + 2MX ₂ I- 4NH.,
-Si (OR) ₄ "i 2MF ₂ +

The theoretical yield of the reaction, based on SiF ₄ , is 100%. In practice, yields of over 90% of theory can be achieved.

With the method according to the invention, a way could be found to shift the equilibrium of reaction (1) with ammonia to the side of the silicic acid ester without reducing the practically achievable ester yield by the formation of fluorosilicate to less than a third of the SiF _{4 used} .

The ester synthesis through the use of ammonia in combination with the metal salts brings further advantages:

Ammonia neutralizes the resulting war-serstolTionen without the formation of water, which would _{undesirably convert SiF 4} to hexafluorosilicic acid and silica;

there are no toxic by-products, such as hydrogen sulfide, which can be further processed or destroyed Need to become;

there are no corrosive by-products such as hydrogen chloride; the reaction can therefore be in simple, carbon steel reactors are carried out.

Suitable alcohols are primary and secondary, saturated and unsaturated aliphatic alcohols as well aromatic alcohols. Examples include methanol, ethanol, propanol, butanol, cyclohexanol, Phenol and cresol.

The process can be carried out at room temperature or at an elevated temperature. Preferably is carried out at room temperature. The reaction can take place at atmospheric pressure or overpressure respectively.

A preferred embodiment of the process consists in that first the anhydrous salt MX or MX ₂ and then 1 mol of ammonia per gram equivalent MX or MX _{2 are dissolved} in excess alcohol. Then ¹ A mole of silicon tetrafluoride per gram equivalent of MX or MX., Is introduced. The batch heats up and NH ₄ X and MF or MF., Precipitate. After the addition of SiF _{4 has} ended, the batch can usually be processed further straight away.

In some cases, higher yields can be achieved if the batch continues for one to three hours is heated under reflux.

The neutral to slightly ammoniacal batch of the precipitated salts can be used for further processing filtered off or centrifuged and the Pillrate or centrifugate fractionated to obtain the ester. However, it is also possible to use the neutral to slightly ammoniacal approach by distillation separate from the salts and then fractionate the distillate to obtain the ester.

The invention is explained in more detail below with the aid of examples. In Example 1 was used as a metal salt Calcium chloride is used, which is a waste product in soda production after the general today the usual Solvay process.

example 1

80 g of dry calcium chloride was dissolved in 600 ml of dry methanol in a reaction vessel equipped with a stirrer, a reflux condenser and a gas line tube. The reflux condenser was protected against the ingress of atmospheric moisture by a calcium chloride tube. Subsequently, 24.5 g of dry ammonia were introduced through the gas line pipe while cooling the batch. A clear solution was obtained. _{Finally, 36.5 g of SiF 4 were} introduced through the gas inlet tube with vigorous stirring. A white precipitate formed. After the introduction of gas had ended, the batch was distilled and the distillate was fractionated. It was 48.4 g

Orthosilicic acid methyl ester, boiling point 120 ° C at 720 torr. The yield was 90.7%, based on the SiF _{4 used} . The distillation residue consisted essentially of calcium fluoride and ammonium chloride. Hexafluorosilicate could not be detected.

Example 2

In the reaction vessel described in Example 1, 35.2 g of dry ammonia were dissolved in 800 ml of dry ethanol with cooling. Then 85 g of lithium chloride were dissolved in the ammoniacal ethanol to form a clear solution. Then 52 g of SiF _{4 were} introduced. A white precipitate was formed and the batch warmed to about 60 ° C. The batch was heated under reflux for two hours, then distilled and the distillate was fractionated. 90.6 g of ethyl orthosilicate, boiling point 166 ° C. at 715 torr, were obtained. The yield was 86.9%, based on the SiF _{4 used} .

Comparative example

In this example, a metal salt was used which did not meet the requirements of the invention becomes right. Contrary to the requirement, the sodium chloride used forms a moderately sparingly soluble one Fluoride, on the other hand a hexafluorosilicate that is practically insoluble in alcohols,

In the reaction vessel described in Example 1, which, however, was still provided with a dropping funnel, 113 g of dry sodium chloride were suspended in 600 ml of dry methanol. 33.4 g of ammonia were passed into the suspension with cooling and then 300 ml of methanol in which 49.5 g of SiF _{4 had been} dissolved were slowly added dropwise with vigorous stirring. The batch heated up to 44 ° C. The mixture was then refluxed for three hours, distilled and the distillate was fractionated. 27.5 g of methyl orthosilicate, ie a yield of 38.0%, based on the SiF _{4 used, were} obtained. The distillation residue consisted of a mixture of sodium fluoride, sodium oxafluorosilicate and ammonium chloride.

The yield here is significantly lower compared to the procedure according to the invention. At 38%, however, it is still twice as high as that of Example 2 of DT-AS 11 30 799, in which work was carried out without the simultaneous use of ammonia. According to this example, only a yield of 15%, based on the SiF _{4 used} , was obtained.

Claims (2)

Patent claims: 1. Process for the preparation of orthosilicic acid esters by reacting silicon tetrafluoride with alcohols in the presence of metal salts, characterized in that the silicon tetrafluoride with an alcohol in the presence of lithium chloride, bromide, nitrate, sulfate or carbonate or calcium chloride, bromide or nitrate or mixtures thereof and Ammonia is converted. 2. The method according to claim 1, characterized in that first the metal salt and then Ammonia are dissolved in the alcohol and then silicon tetrafluoride is introduced with stirring will. so that this method as a whole according to the gross orcaklion equation (4) expires: