Process for preparing trimethylchlorosilane

Abstract

A process for preparing trimethylchlorosilane (M3) and methyltrichlorosilane (M1) by disproportionation of dimethyldichlorosilane (M2) in the presence of an Al.sub.2O.sub.3 catalyst is described herein. The dimethyldichlorosilane used is in the form of a silane mixture that includes 80-100% by weight of dimethyldichlorosilane (M2). The difference in content from 100% by weight includes M1 and M3.

Claims

1. Process for preparing trimethylchlorosilane (M3) and methyltrichlorosilane (M1) by disproportionation of dimethyldichlorosilane (M2) in the presence of an Al.sub.2O.sub.3 catalyst, wherein the Al.sub.2O.sub.3 catalyst utilized has a BET surface area within a range of 100-200 m.sup.2/g, the dimethyldichlorosilane being used is in the form of a silane mixture that comprises 80-100% by weight of dimethyldichlorosilane (M2), and wherein the difference in content from 100% by weight comprises M1 and M3.

2. The process according to claim 1, wherein the catalyst is activated prior to the disproportionation by passing (i) SiCl.sub.4, or (ii) HCl gas; or (iii) a mixture of HCl gas and at least one chlorosilane of the formula (II)
R.sub.xSiCl.sub.4-x (II), where the radicals R are independently selected from the group consisting of (i) hydrogen and (ii) C.sub.1-C.sub.5 alkyl radical, and the index x has the values 0, 1, 2 or 3, over the catalyst at a temperature within a range from 330 C. to 550 C.

3. The process according to claim 2, wherein the chlorosilane is M2 or SiCl.sub.4.

4. The process according to claim 2, wherein the catalyst is activated prior to the disproportionation by passing HCl gas over the catalyst.

5. The process according to claim 1, wherein an Al.sub.2O.sub.3 catalyst having a pore volume within a range of 0.1-1 cm.sup.3/g is used.

6. The process according to claim 1, wherein a -Al.sub.2O.sub.3 catalyst is used.

7. The process according to claim 1, wherein the silane mixture comprises 98-100% by weight of M2.

8. The process according to claim 1, wherein the process is executed continuously.

Description

DETAILED DESCRIPTION

(1) This object is achieved by the process of the invention .

(2) The process of the invention provides very good yields of M3 that are close to the thermodynamic maximum. Moreover, M1 is formed in stoichiometric amounts, which indicates that no side reactions or decomposition reactions occur.

(3) The present invention provides a process for preparing trimethylchlorosilane by disproportionation of dimethyldichlorosilane (M2) in the presence of an Al.sub.2O.sub.3 catalyst, the dimethyldichlorosilane being used in the form of a silane mixture that comprises at least 80% by weight of dimethyldichlorosilane (M2), and wherein the difference in content from 100% by weight comprises M1 and M3.

(4) Silane disproportionationstrictly speaking this should be termed dismutationmeans that an individual silane rearranges to two or more different silanes (e.g. M2->M3+M1), wherein the substituents in the silanes formed (e.g. Cl and CH.sub.3) have a substitution pattern different to that of the original silane. A silane disproportionation reaction can for example be illustrated by formula (I) below:
2 (CH.sub.3).sub.2SiCl.sub.2.Math.CH.sub.3SiCl+(CH.sub.3).sub.3SiCl (I)

(5) The position of the thermodynamic equilibrium in this disproportionation of dimethyldichlorosilane is normally at high concentrations of dimethyldichlorosilane. In the absence of a catalyst, the disproportionation reaction occurs very slowly or not at all; with the catalysts and conditions of the prior art, the reaction is likewise still too slow or does not achieve the thermodynamically possible yield of M3.

(6) M2 is used in the form of a silane mixture that comprises 80-100% by weight of dimethyldichlorosilane (M2), and wherein the difference in content from 100% by weight comprises M1 and M3.

(7) The silane mixture preferably comprises 98-100% by weight of M2, most preferably the silane mixture comprises 99.5-100% by weight of M2. The difference in content preferably exclusively comprises M1.

(8) The Al.sub.2O.sub.3 catalyst is preferably activated prior to the disproportionation by passing (i) SiCl.sub.4, or (ii) HCl gas; or (iii) a mixture of HCl gas and at least one chlorosilane of the formula (II)
R.sub.xSiCl.sub.4-x (II), where the radicals R are independently selected from the group consisting of (i) hydrogen and (ii) C.sub.1-C.sub.5 alkyl radical, and the index x has the values 0, 1, 2 or 3,

(9) over the catalyst at a temperature within a range from 330 C. to 550 C.

(10) The radicals R in the formula (II) are preferably independently selected from methyl radical or hydrogen.

(11) Examples of chlorosilanes of the formula (II) are Me.sub.2SiCl.sub.2, Me.sub.3SiCl, MeSiCl.sub.3, Me.sub.2ClSiH, Cl.sub.3SiH, SiCl.sub.4.

(12) The Al.sub.2O.sub.3 catalyst is particularly preferably activated prior to the disproportionation by passing HCl gas over the catalyst.

(13) The contact time in the activation is normally 1-100 seconds.

(14) Preference is given to using an Al.sub.2O.sub.3 catalyst having a BET surface area within a range of 100-200 m.sup.2/g, more preferably within a range of 150-180 m.sup.2/g. The BET specific surface area is the specific surface area determined by adsorption of nitrogen in accordance with standard ASTM D 3663-78, which is based on the Brunnauer-Emmet-Teller method (J. Am. Chem. Soc. 1938, 60, 309-319).

(15) The Al.sub.2O.sub.3 catalyst used preferably has a pore volume within a range of 0.1-1 cm.sup.3/g, more preferably within a range of 0.4-0.5 cm.sup.3/g.

(16) The pore volume can be determined for example by mercury porosimetry.

(17) The Al.sub.2O.sub.3 catalyst may contain up to 20% by weight of other elements, e.g. carbon, or smaller amounts of silicon and/or chlorine. For example, residues of auxiliary substances used for shaping may thus be present.

(18) The Al.sub.2O.sub.3 catalyst is normally present in the form of a shaped body. Examples of shaped bodies are tablets, granules, spheres, rings, cylinders, hollow cylinders.

(19) The shaped bodies preferably have a size within a range of 1-10 mm, more preferably within a range of 2-4 mm.

(20) Particular preference is given to using a -Al.sub.2O.sub.3 catalyst. Most preferably, a -Al.sub.2O.sub.3 catalyst in the form of a shaped body having a BET surface area within a range of 100-200 m.sup.2/g, a pore volume within a range of 0.1-1 cm.sup.3/g and a size within a range of 1-10 mm is used.

(21) The process of the invention is particularly preferably executed at a temperature within a range from 390 C. to 490 C. and a pressure of less than 1 bar.

(22) The process may be executed continuously or batchwise, with a continuous process being preferable. With the continuous operation of the process, M2 can be separated from the product mixture by distillation and fed back into the process.

(23) The contact time for the silane mixture is normally within a range of 0.1-120 seconds, preferably within a range of 1-30 seconds, more preferably the contact time is within a range of 10-15 seconds.

(24) At the end of the reaction, the trimethylchlorosilane can be separated from the reaction mixture, e.g. by distillation.

EXAMPLES

(25) The experiments were carried out in a tubular reactor filled with Al.sub.2O.sub.3 (Al 3438 T pellets, BASF). The reactor was heated with a heating jacket; the heating zone filled with catalyst had a height of approx. 30 cm and a diameter of 5 cm. The M2 was first vaporized and preheated before it was able to come into contact with the catalyst. The product mixture was condensed and collected by means of a reflux condenser.

(26) GC measurements were performed using an Agilent 6890N (WLD detector; columns: HP5 from Agilent: length: 30 m/diameter: 0.32 mm/film thickness: 0.25 m; RTX-200 from Restek: length: 60 m/diameter: 0.32 mm/film thickness: 1 m). Retention times were compared with the commercially available substances, all chemicals were used as purchased. All values are in percent by weight.

Example 1

(27) The catalyst was activated with vaporized SiCl.sub.4 at 450 C. (internal measurement in the centre of the bed). 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 378 C. and with a contact time in the reactor of 20 s, and the condensed product mixture was analysed.

(28) The product mixture consisted of 76.9% by weight of M2, 13.3% by weight of M1 and 9.6% by weight of M3. Small amounts of unidentified by-products were present.

Example 2

(29) With the same catalyst load as in example 1, 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 377 C. and with a contact time in the reactor of 30 s, and the condensed product mixture was analysed.

(30) The product mixture consisted of 74.1% by weight of M2, 15.4% by weight of M1 and 10.2% by weight of M3. Small amounts of unidentified by-products were present.

Example 3:

(31) With the same catalyst load as in example 1, 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 485 C. and with a contact time in the reactor of 7 s, and the condensed product mixture was analysed.

(32) The product mixture consisted of 75.1% by weight of M2, 14.2% by weight of M1 and 10.7% by weight of M3. Small amounts of unidentified by-products were present.

Example 4

(33) The catalyst was activated with HCl gas at 450 C. (internal measurement in the centre of the bed). 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 390 C. and with a contact time in the reactor of 25 s, and the condensed product mixture was analysed.

(34) The product mixture consisted of 77.9% by weight of M2, 12.8% by weight of M1 and 9.3% of M3. Small amounts of unidentified by-products were present.

Example 5

(35) The catalyst was activated with a mixture of HCl gas and vaporized SiCl.sub.4 at 450 C. (internal measurement in the centre of the bed). 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 400 C. and with a contact time in the reactor of 18 s, and the condensed product mixture was analysed. The product mixture consisted of 78.0% by weight of M2, 12.6% by weight of M1 and 9.4% by weight of M3. Small amounts of unidentified by-products were present.

Example 6

(36) 20 g of catalyst (activated beforehand with HCl) and 150 mL of M2 were heated to 350 C. for 3 hours in a closed autoclave. After cooling, the autoclave was opened and the contents analysed.

(37) The product mixture consisted of 76.8% by weight of M2, 14.1% by weight of M1 and 9.1% by weight of M3. Small amounts of unidentified by-products were present.

Comparative Example 1

(38) With an unactivated catalyst load, 150 mL of vaporized M2 was pumped through the catalyst bed at a reactor internal temperature of 350 C. and with a contact time in the reactor of 60 s, and the condensed product mixture was analysed.

(39) The product mixture consisted of 68.3% by weight of M2, 14.4% by weight of M1 and 7.7% by weight of M3. Also present was a further 8.4% by weight of partially chlorinated methylated disiloxanes that cannot be used further.

(40) The examples show that both the yield of M3 and the amount of M1 formed are much better than the values achieved in the prior art. They also correspond to the thermodynamic equilibrium cited in EP0219718.