1. Field of the Invention
The invention relates to a continuous process for preparing SiOC-containing compounds, preferably for preparing alkoxysilanes and alkoxy-rich silicone resins.
2. Description of the Related Art
This process, which is usually referred to as “alkoxylation”, proceeds from the corresponding chlorosilanes, the silicon-bonded chlorine atoms of which are exchanged for alkoxy groups. This is possible through a reaction of the chlorosilanes with the particular alcohol or else an alcohol-water mixture with release of hydrochloric acid. If the pure alcohol is used, this gives alkoxysilanes, whereas the use of alcohol-water mixtures leads to the formation of alkoxy-functional siloxanes. In this case, the mean molecular mass of the siloxanes formed can be varied virtually as desired via the water content of the alcohol-water mixtures used.
For reasons of cost, it is desirable to perform the alkoxylation in a continuous operation. Corresponding processes are already known and are described, for example, in EP 1205505 or EP 1686132, and the references cited therein.
These alkoxylation processes are typically performed in plants consisting of a (pre)reactor and one or two columns for a reactive distillation. FIG. 1 shows a schematic diagram of the plant for the continuous alkoxylation process described in EP 1686132. Column 2 which is shown in FIG. 1 is optional in principle, but the use of the second column is advantageous for attainment of a product quality containing only a small proportion of silicon-bonded chlorine atoms.
In such a plant, alcohol and chlorosilane are contacted with one another countercurrently. Thus, the chlorosilane is first contacted in an upstream reactor with a usually substoichiometric amount of alcohol based on the chlorine atoms, forming a partly alkoxylated silane product and gaseous hydrogen chloride. The alcohol can be added directly or else can originate (in a form with a higher or lower hydrochloric acid content) from column 1 or else column 2. The hydrogen chloride is removed as an offgas from the plant, whereas the silane is transferred into the column 1 and fed into the upper column part thereof or even at the top of the column.
In column 1, the silane flows from the top downward, whereas the particular alcohol is vaporized by means of a circulation vaporizer at the foot of the column and is moved in the opposing direction in gaseous form. The column 1 offers a large surface area between gaseous and liquid phases through insertions of random packing, and so there is intense contact between silane and alcohol. It is thus possible to exchange the silicon-bonded chlorine atoms still present for alkoxy groups. The silane having a distinctly lower chlorine level is finally removed from the distillation pot of the circulation vaporizer, whereas the alcohol, which is accordingly in hydrochloric acid solution, is obtained at the top of column 1.
The acidic alcohol can be passed on into the reactor, whereas the silane, according to the purity, can be utilized directly as the end product or else transferred into column 2.
Column 2 works by an identical principle to column 1 and serves—if necessary—for further depletion of silicon-bonded chlorine atoms in the silane phase. The alcohol-hydrochloric acid mixture obtained via the top can be passed on into column 1 or else alternatively directly into the reactor.
In this case, it is possible to use the entire amount of alcohol required for the operation at the foot of column 2 and to conduct it counter to the silane stream through both reaction columns. However, it is likewise also possible to add a portion of the process alcohol at other points, for example at the foot or else various metering points in column 1, or not until within the reactor.
It will be appreciated that it is likewise possible to conduct this operation not with the pure alcohol but instead with an alcohol-water mixture. In this case, the water substitutes as many as two chlorine atoms on different silicon atoms, such that a siloxane unit is the ultimate result. Instead of monomeric alkoxysilanes, oligomeric siloxanes rich in alkoxy groups are thus obtained. However, this does not change anything about the principle of operation detailed above.
In this continuous operation, it is desirable to attain maximum conversion, or as already stated, to obtain a product containing only a minimum number of silicon-bonded chlorine atoms, if any. Thus, chlorine atoms remaining on the silicon atom would be easily substitutable by other compounds having active hydrogen, and there would therefore be a release of chloride or hydrochloric acid in downstream synthesis stages and/or applications of the particular products, and this would almost always have a massively disruptive effect. There are known processes for reprocessing of alkoxysilanes and/or alkoxysiloxanes having chloride radicals remaining on the silicon, for example from EP 0999215 or EP 0997468, but any reprocessing operations are associated with additional operating steps. For reasons of cost, it is therefore always advantageous when the alkoxylation operation leads directly to alkoxysilanes and/or alkoxysiloxanes with a sufficiently low content of silicon-bonded chlorine atoms.
As well as very substantially complete removal of the silicon-bonded chlorine, it is of course likewise desirable to use the alcohol required for this purpose or the alcohol-water mixture required for this purpose only in a slightly superstoichiometric amount, if at all, and accordingly to obtain very substantially pure hydrogen chloride as the coproduct. This is all the more true in that the alcohol used in excess is of course obtained not in pure form but as a mixture with hydrogen chloride. In this acidic alcohol, there is nucleophilic substitution of the alcoholic hydroxyl group by the chloride even at room temperature. In other words, the corresponding chloroalkanes and water are formed. Workup of these complex mixtures of alcohol, chloroalkane, water and hydrogen chloride with the aim of obtaining reusable materials of value is usually very complex and not very cost-effective.
The aim of arriving at sufficiently low-chlorine alkoxysilanes without any great alcohol excesses, however, is achieved in the processes according to the prior art only for a few selected silanes. Especially silanes having relatively large silicon-bonded alkyl groups can be alkoxylated only extremely incompletely by these processes and/or require great alcohol excesses.
The problem addressed was therefore that of developing a process for alkoxylating chlorosilanes, which has these disadvantages of the processes according to the prior art only to a distinctly reduced degree, if at all.