GB1559335A - Process for producing orthosilicic acid tetra alkyl esters - Google Patents

Description

(54) PROCESS FOR PRODUCING ORTHOSILICIC ACID TETRA ALKYL ESTERS (71) We, Dynamit Nobel Aktiengesellschaft, a Germany Company of 521 Troisdorf, bez Koln, Postfach 1209, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing orthosilicic acid alkyl esters.

It is known from German patent specification 1 768 781 to produce orthosilicic acid tetraethyl esters by reacting metallic silicon with ethanol in the presence of concentrated alkali metal ethylate solutions; however this method has the disadvantage that it achieves relatively low space-time yields. This disadvantage is reduced to a certain extent by the method of British Patent specification 1 237 159, wherein the reaction is carried out in the presence of at least 70% by weight of the desired orthosilicic acid tetraalkyl ester, which results necessarily in a lower concentration of the alkali metal alcoholate in the reaction mixture.

Both methods are very well suited to the production of the silicic acid tetramethyl ester. However problems arise with the production of silicic acid tetraethyl ester and higher esters, which problems result in a reduction in the material yield and the space-time yield; the causes of these problems are still unexplained. In the production of silicic acid tetraethyl ester and higher esters, the reaction rate with respect to the silicon put in at the outset decreases steadily during the course of the reaction without the reaction constituents being reacted to completion. This reduction in the reaction rate is quicker than the reduction in the concentration of the constituents. In the case of the production of silicic acid tetraethyl ester, this results, for instance, in only about 40% of the silicon used reacting, the reaction then ceasing despite a surplus of ethanol.

According to the present invention there is provided a process for producing an orthosilicic acid tetraalkyl ester each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, which process is carried out under pressure at a temperature which is greater than the normal boiling temperature of the reaction mixture, to form the desired ester.

By normal boiling temperature there is meant the boiling temperature at one atmosphere absolute.

Preferably the ester produced is the ethyl ester, in which case the alkanol is ethanol and the alkali metal alkoxide is the ethoxide, preferably sodium ethoxide, also known as sodium ethanolate.

During the reaction between silicon and the alkanol, nascent hydrogen is formed, which hydrogenates the alkanol to a small degree, forming alkanes and water, (c.f.

Houben-Weyl VI/2, page 100). This water reacts with the catalyst present, i.e. the alkoxide, which thereby suffers reduced activity, and also with the ester formed in the alkaline reaction medium. Under the reaction conditions specified there is an increase in the rate of reaction, which is contrary to the result expected from thermodynamic considerations. Thus it would normally have been expected that the hydrogen gas, which is not removed during the reaction, would displace the reaction equilibrium towards the reactants rather than the products. Surprisingly, however, this has been found not to be the case, even though the reaction of the nascent hydrogen with the alkanol to form water is more pronounced with increasing pressure.

Theoretically, this increased concentration of water should inhibit the increase in the rate of reaction between the silicon and the alkanol under pressure and at high temperature. In practice, however, the opposite has been found to be the case.

The reaction between silicon and the alkanol in the presence of the alkali metal alkoxides generally starts at about 1600C.

Up to this temperature, operating under pressure, the system can be kept closed.

After the reaction has started a further external addition of heat is not necessary since the reaction is exothermic. Preferably the reaction is carried out at a temperature of from 160 to 210 C. Once the reaction commences, the rise in pressure is no longer proportional to the rise in temperature; instead excess pressure is generated as a result of the hydrogen produced during the reaction. It is preferred to carry out the reaction at an excess pressure of up to 50 atmospheres absolute. In general the pressure at which the process is carried out will depend on the apparatus available. When the desired overpressure is reached, it can easily be kept constant by discharging the newly produced hydrogen via a suitable valve. The discharge of the hydrogen also makes control of the reaction temperature possible since an alkanol silicic acid ester mixture also escapes when the hydrogen is vented, thereby removing heat of evaporation from the system.

The ability to discharge the alkanol/silicic acid ester mixture also makes it possible to carry out the process continuously. Further amounts of elemental silicon or of the alloy containing silicon are added to replace the material which has reacted, preferably dosed in as a dispersion in the alkanol or in the silicic acid ester. The amount of alkanol added and the ratio of silicon to alkanol is dependent on the amount of alkanol/silicic acid ester mixture drawn off. A prerequisite for a continuous process is the establishment of a temperature at which the reaction mixture boils under the selected pressure. The boiling point of the mixture is governed by the alkanol content thereof.

In principle, the process may be performed in the manner disclosed in German Patent Specification 17 68 781 or British Patention Specification 1 237 159. Thus with reference to the latter specification, there is preferably present at all times during the reaction at least 70% by weight of the orthosilicic acid tetraalkyl ester, based on the total weight of liquid present during the reaction. The concentration of the individual components in the system can be varied within wide limits. It is an advantage, however, to select the proportions so that they give a mixture which is capable of being easily stirred. It is preferred, too, that the alkoxide present becomes dissolved in the alkanol. It is also possible to carry out the process without excess silicon or alloy in the system, and to supply the silicon or alloy together with the alkanol only at the rate at which it reacts.

The process may be carried out in batches or continuously. Preferably the separation of the ester product from the product mixture is carried out by distillation.

The water which is produced in small quantities by the secondary reaction mentioned previously reacts, in the alkaline medium, with the alkoxide catalyst, thereby reducing its activity, and also with the silicic acid ester which is formed. The removal of this water is preferably effected by partially distilling the reaction mixture, using the hydrogen which is produced or an inert gas, for example nitrogen, as an entraining gas. The aqueous, gaseous distillation products are condensed in a manner such that the condensate cannot run back directly into the reaction vessel. Thereafter the distillate may be returned to the remainder of the reaction mixture in the reaction vessel via suitable dehydrating agents.

In addition to elemental silicon, it is possible to use an alloy containing silicon in the process according to the invention. Preferably there is used ferro-silicon or other silicon alloys with a proportion of silicon above 50%. The grain size of the silicon or alloys used is preferably not greater than 100 ,u, and more preferably from 2 to 20 ,u.

Example 2 of the following Examples illustrates the invention.

Example 1 (Comparison Example) There was provided a one litre vessel having a wall-travelling anchor stirrer and a temperature measuring device, which vessel had a reflux condenser with an open end from which the hydrogen produced during the reaction was removed and measured by means of a gas meter. In the vessel were placed 200 g of FeSi having a grain size from 15 to 20 ,u, 400 g of silicic acid tetraethyl ester and 20 g of sodium ethylate. Thereafter 100 g of ethyl alcohol was added and the mixture was heated to reflux temperature at atmospheric pressure.

When the reflux temperature had been reached, hydrogen was evolved at a rate of approximately 6 1 per hour, this being equivalent to a space-time yield of 28.5 g of silicic acid tetraethyl ester/ 1/h.

Example 2 There was provided a heatable, one litre, stirrer-equipped autoclave having a temperature measuring device and a pressure resistant reflux condenser, at the upper end of which condenser was located a regulating valve, for adjusting the pressure and discharging the hydrogen generated. Into the autoclave, which was preheated to 1500C, were placed 200 g FeSi having a grain size from 15 to 20 ,u, 400 g silicic acid tetraethyl ester, 20 g sodium ethylate and 100 g ethyl alcohol. When the above additions had been made the heat supply was switched on and the temperature rose to 185"C within 5 minutes, the pressure rising to approximately 10 atmospheres absolute. This pressure was kept constant by removing the further amounts of hydrogen which evolved via the regulating valve. The quantity of gas evolved was then measured.

The reaction was continued for a further 20 minutes, during which time the temperature rose to 1980C. At the end of this period, a total volume of 20 1 of hydrogen had been discharged, corresponding to a space-time yield of about 280 g silicic acid tetraethyl ester/ 1/h.

This result demonstrates that the process according to the invention is capable of resulting in a considerably increased spacetime yield in comparison with the known method as carried out in Example 1.

It is a preferred feature of the present invention that the process be carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction. It is also preferred that the process is additionally or alternatively carried out in the presence of a surface-active agent.

Our copending divisional patent Application No.79/01185 (Serial No. 1559336) describes and claims a process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction, to form the desired ester.

Our other copending divisional patent application No. 79/01276 (Serial No.1559337) describes and claims a process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out in the presence of a surface-active agent, to form the desired ester.

WHAT WE CLAIM IS: 1. A process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out under pressure at a temperature which is greater than the normal boiling temperature of the reaction mixture, to form the desired ester.

2. A process according to claim 1 wherein the reaction is carried out at a temperature of from 160 to 210 C.

3. A process according to claim 1 or 2 wherein the reaction is carried out at an excess pressure of up to 50 atmospheres absolute.

4. A process according to claim 1, 2 or 3 wherein the pressure is adjusted by venting the hydrogen which is produced during the course of the reaction.

5. A process according to any one of the preceding claims which is carried out continuously and wherein there is produced a reaction mixture from which is removed the desired ester and hydrogen, and to which is added further elemental silicon or alloy containing silicon.

6. A process according to claim 5 wherein the further elemental silicon or alloy containing silicon is added in the form of a dispersion in the alkanol or in the ester.

7. A process according to any one of the preceding claims wherein part of the reaction mixture is removed from the reaction vessel by distillation, dehydrated and then admixed with the remainder of the reaction mixture.

8. A process according to any one of the preceding claims wherein the alkyl group of the orthosilicic acid tetraalkyl ester and the alkanol is an ethyl group.

9. A process according to any one of the preceding claims wherein the elemental silicon or alloy containing silicon has a grain size of from 2 to 20 E.L.

10. A process according to any one of the preceding claims wherein the alloy containing silicon is ferrosilicon.

11. A process according to any one of the preceding claims which is carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction.

12. A process according to any one of the preceding claims which is carried out in the presence of a surface-active agent.

13. A process according to any one of the preceding claims wherein there is present at all times during the reaction at least 70% by weight of the orthosilicic acid tetraalkyl ester, based upon the total weight of liquid

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. alcohol. When the above additions had been made the heat supply was switched on and the temperature rose to 185"C within 5 minutes, the pressure rising to approximately 10 atmospheres absolute. This pressure was kept constant by removing the further amounts of hydrogen which evolved via the regulating valve. The quantity of gas evolved was then measured. The reaction was continued for a further 20 minutes, during which time the temperature rose to 1980C. At the end of this period, a total volume of 20 1 of hydrogen had been discharged, corresponding to a space-time yield of about 280 g silicic acid tetraethyl ester/ 1/h. This result demonstrates that the process according to the invention is capable of resulting in a considerably increased spacetime yield in comparison with the known method as carried out in Example 1. It is a preferred feature of the present invention that the process be carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction. It is also preferred that the process is additionally or alternatively carried out in the presence of a surface-active agent. Our copending divisional patent Application No.79/01185 (Serial No. 1559336) describes and claims a process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction, to form the desired ester. Our other copending divisional patent application No. 79/01276 (Serial No.1559337) describes and claims a process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out in the presence of a surface-active agent, to form the desired ester. WHAT WE CLAIM IS:

1. A process for producing an orthosilicic acid tetraalkyl ester, each alkyl group of which has a carbon chain containing from 2 to 6 carbon atoms, which process comprises reacting elemental silicon or an alloy containing silicon with an alkanol having a carbon chain containing from 2 to 6 carbon atoms in the presence of an alkali metal alkoxide, the alkyl group of which is identical with the alkyl group of the alkanol, the reaction being carried out under pressure at a temperature which is greater than the normal boiling temperature of the reaction mixture, to form the desired ester.

2. A process according to claim 1 wherein the reaction is carried out at a temperature of from 160 to 210 C.

3. A process according to claim 1 or 2 wherein the reaction is carried out at an excess pressure of up to 50 atmospheres absolute.

4. A process according to claim 1, 2 or 3 wherein the pressure is adjusted by venting the hydrogen which is produced during the course of the reaction.

5. A process according to any one of the preceding claims which is carried out continuously and wherein there is produced a reaction mixture from which is removed the desired ester and hydrogen, and to which is added further elemental silicon or alloy containing silicon.

6. A process according to claim 5 wherein the further elemental silicon or alloy containing silicon is added in the form of a dispersion in the alkanol or in the ester.

7. A process according to any one of the preceding claims wherein part of the reaction mixture is removed from the reaction vessel by distillation, dehydrated and then admixed with the remainder of the reaction mixture.

8. A process according to any one of the preceding claims wherein the alkyl group of the orthosilicic acid tetraalkyl ester and the alkanol is an ethyl group.

9. A process according to any one of the preceding claims wherein the elemental silicon or alloy containing silicon has a grain size of from 2 to 20 E.L.

10. A process according to any one of the preceding claims wherein the alloy containing silicon is ferrosilicon.

11. A process according to any one of the preceding claims which is carried out in the presence of a compound containing a methoxy group which is not an alkali metal 2-methoxy ethoxide or 2-methoxy ethanol and which increases the rate of reaction.

12. A process according to any one of the preceding claims which is carried out in the presence of a surface-active agent.

13. A process according to any one of the preceding claims wherein there is present at all times during the reaction at least 70% by weight of the orthosilicic acid tetraalkyl ester, based upon the total weight of liquid

present during the reaction.

14. A process according to claim 1 sub stantially as described with reference to Example 2.

15. An orthosilicic acid tetraalkyl ester whenever produced by the process according to any one of the preceding claims.