Isolation of trimethoxysilane from a TMS/methanol mixture

Details Isolation of trimethoxysilane from a TMS/methanol mixture Isolation of trimethoxysilane from a TMS/methanol mixture

2000-10-03

2001-07-03

Shaver, Paul F. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Silicon containing

C556S466000

Reexamination Certificate

active

06255514

ABSTRACT:

The invention relates to a process for isolating trimethoxysilane (TMS) from a TMS/methanol mixture.
TMS is customarily prepared by direct synthesis from silicon metal and methanol. In this process, TMS is obtained in the form of TMS/methanol mixtures. These mixtures cannot be worked up by simple distillation because the two components TMS and methanol form an azeotrope. The TMS/methanol azeotrope consists of 55% by weight of TMS and 45% by weight of methanol and boils at 62.5° C., while methanol has a boiling point of 64.5° C. and TMS has a boiling point of 84° C.
Processes for working up TMS/methanol mixtures are known from the prior art. In these, the TMS/methanol mixture is separated by means of extractive distillation with the aid of azeotrope breakers such as n-hexane, n-heptane or organic solvents in general which have a dipole moment of <1.61 D and form an azeotrope with methanol. The fact that the azeotrope breakers form an azeotrope with methanol makes these processes disadvantageous, since the recovery and reuse of the individual components methanol and the azeotrope breaker is very complicated and thus uneconomical.
EP-A 0 310 920 discloses a process for separating TMS from a TMS/methanol mixture by extractive distillation. In this process, high-boiling tetramethoxysilane (TTMS), which is unreactive and does not form an azeotrope with either of the components of the mixture, is used as extractant. Extraction occurs because of the greater affinity of TTMS for TMS than for methanol. However, a good separation requires a 3- to 10-fold excess of TTMS which has to be prepared separately, which means that the costs of this process are high.
EP-A 0 462 359 relates to a process for working up a TMS/methanol mixture without addition of further substances. In this process, the TMS/methanol azeotrope obtained is returned to the synthesis reactor for the direct synthesis of TMS from silicon metal and methanol and only the amount of TMS above the azeotrope ratio of TMS to methanol is isolated.
All the abovementioned processes have the disadvantage that the TMS/methanol mixture has to be brought to elevated temperatures without prior removal of the methanol, as a result of which the undesirable subsequent reaction of TMS with methanol to give tetramethoxysilane (TTMS) is accelerated and the yield of TMS is therefore reduced.
JP 60/252488 (as Derwent Abstract No. 86-031969/05) relates to the extraction of TMS from a TMS/methanol mixture using organic solvents and/or organopolysiloxanes which are immiscible with the TMS/methanol mixture but are miscible with the TMS. An inorganic or organic salt can be added for salting-out, so as to improve the effectiveness of the extraction. The best TMS/methanol ratios achieved in the extract phase (=uptake phase, essentially solvent and desired product, here TMS) are 93.7% by weight of TMS:6.3% by weight of methanol.
According to K. Sattler, “Thermische Trennverfahren”, second revised and expanded edition, VCH-Verlagsgesellschaft mbH, Weinheim, 1995, pages 498 ff, liquid-phase extraction does not lead, in contrast to other possibly competing separation processes such as distillation, directly to the individual, separated components of the mixture, but instead an additional separation step is necessary. This is because the raffinate phase (=release phase, consisting essentially of a carrier material, here methanol) comprises not only the carrier material but also residues of solvents which may have to be separated off. The extract phase (=uptake phase, essentially solvent and desired product, here TMS) consists essentially of solvents and desired product and therefore has to be separated into the solvent to be returned to the extraction step and the desired product in an additional separation step. This increased complexity of an extraction compared to a distillation is only economical when very good separation of the components of the mixture is achieved, i.e. when a solvent having a high selectivity is used.
A critical factor for carrying out the extraction process for separating TMS and methanol is therefore the discovery of a suitable solvent or a suitable way of achieving a high separation efficiency. The choice of an optimum solvent for the extraction makes it possible to achieve cost savings and thus good economics of the extraction process.
It is an object of the present invention to provide an economical process for isolating TMS from a TMS/methanol mixture, in which process TMS can be obtained in good yields without thermal stressing of the TMS/methanol mixture.
The achievement of this object starts out from an extraction process for isolating TMS from a TMS/methanol mixture by formation of two phases, namely a TMS-rich extract phase and a methanol-rich raffinate phase, by addition of one or more methanol-soluble inorganic and/or organic salts and, if desired, an additional nonpolar organic solvent and subsequent phase separation.
In the process of the present invention, the amount of salt added is selected so that the weight ratio of TMS to methanol in the extract phase is at least 94% by weight (TMS):6% by weight (methanol) (normalized to 100% by weight).
The use of the optimum extractant enables very good separation efficiencies to be achieved, as a result of which TMS can be isolated in pure form in high yields. The process of the present invention makes it possible to isolate TMS gently without thermal stressing of the TMS/methanol mixture, so that the formation of by-products is low.
For the purposes of the present invention, an extraction process is a process in which a separation of the TMS/methanol mixture to form two phases, namely a TMS-rich phase (extract phase) and a methanol-rich phase (raffinate phase), is carried out. In this process, the addition of a nonpolar solvent is possible but not absolutely necessary.
The higher the ratio of TMS to methanol in the extract phase, the higher the yield of TMS. The ratio of TMS to methanol is therefore preferably ≧96% by weight:≦3% by weight, particularly preferably ≧98% by weight:≦2% by weight.
The amount of salt which is optimum in terms of separation efficiency and economics depends, inter alia, on whether additional nonpolar solvent is added and, if so, which nonpolar solvent is added. When additional nonpolar solvent is added, the amount of salt, based on the amount of TMS and methanol, is preferably at least 0.5% by weight, particularly preferably from 0.5 to <10% by weight, very particularly preferably from 7 to <10% by weight.
If a nonpolar solvent is used, it serves, together with the salt used, as phase former to form two phases, namely the extract phase consisting essentially of TMS and the nonpolar solvent and the raffmate phase consisting essentially of methanol and salt.
As nonpolar solvents which can be used if desired, preference is given to using nonpolar solvents which are immiscible with the TMS/methanol mixture but are miscible with TMS. Suitable solvents are preferably selected from the group consisting of branched and unbranched C
8
-C
16
-alkanes and mixtures thereof, fluorinated hydrocarbons, diphenylalkanes, dialkylbenzenes and linear alkylbenzenes. Particular preference is given to dodecane and C
12-14
-alkane mixtures.
Any nonpolar solvent added is generally added in a ratio of nonpolar solvent to TMS and methanol of 10-0.5:1, preferably 5-1:1. In a very particularly preferred embodiment of the process of the present invention, no nonpolar solvent is added as extractant.
This embodiment has the advantage that only few components participate in the isolation of the TMS from the TMS/methanol mixture. If the extraction process is followed by purification of the product by distillation, in which large amounts of the nonpolar solvent usually have to be heated so as to separate it off, which is associated with high energy consumption, this embodiment of the present invention requires a significantly lower energy input since no nonpolar solvent has to be separated off. Furthermore, contamination of the TMS by solvent

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