Method of preparing alkoxy silanes

Higher alkoxysilanes are prepared efficiently and in high yield by transesterification of lower C.sub.1-4 alkoxysilanes with C.sub.6-38 higher alcohols at reduced pressure with continuous removal of lower alcohol. The products have little or no lower alkoxy content, and are free of coloring impurities.

TECHNOLOGICAL FIELD 
The invention relates to a process for the preparation of alkoxysilanes by 
transesterisifaction of alkoxy silanes having alkoxy radicals containing 1 
to 4 carbon atoms with higher alcohols in the presence of acid or basic 
catalysts under a reduced pressure. 
DESCRIPTION OF THE RELATED ART 
U.S. Pat. No. 2,643,263 (California Research Co.; issued on Jun. 23, 1953) 
describes tetraalkoxysilanes which, inter alia, are prepared by reacting 
silicon tetrachloride with branched alcohols containing 6 to 18 carbon 
atoms. The preparation starting from silicon tetrachloride has the 
disadvantage that the products obtained show, without further involved 
aftertreatment, a high acidity which leads to undesi red hydrolysis of the 
products during storage. 
WO 93/16085 (Henkel KGaA; issued on Aug. 19, 1993) discloses a process for 
the preparation of light-coloured tetraalkoxysilanes, tetramethoxysilane 
or tetraethoxysilane being transesterified in a 1st step with primary 
higher alcohols in the presence of basic catalysts and, in a subsequent 
step, a neutralization and bleaching by means of solid acidic bleaching 
earths and cation exchangers being carried out. The 2nd step, which 
preferably lasts 1 to 2 hours, is necessary in this process since during 
the 1st step, undesirable, dark-colored tetraalkoxysilanes, which are 
caused by the procedure, are formed. Furthermore, the process has the 
disadvantage that the 1st step is very time-consuming since, caused by the 
procedure, the reaction mixture can be heated only at a heating rate of 
0.1.degree. to 0.5.degree. C. per minute in order to achieve a high degree 
of transesterification and, furthermore, relatively high end temperatures 
are necessary. A further disadvantage of the process is that the 
transesterification takes place in a molar ratio between 
tetramethoxysilane or tetraethoxysilane and the primary alcohols of 
preferably 1:4.2 to 1:4.8 and a further expensive purification step by 
distillation is thus necessary due to the high boiling points. 
SUMMARY OF THE INVENTION 
The invention relates to a process for the preparation of alkoxysilanes 
having alkoxy radicals containing 6 to 38 carbon atoms by 
transesterification of alkoxysilanes whose alkoxy groups contain 1 to 4 
carbon atoms with linear and/or branched higher alcohols containing 6 to 
38 carbon atoms in the presence of basic or acid catalysts under a 
pressure of 1 to 800 hPa and at a temperature of 25 to 150.degree. C. with 
continuous removal of the resulting alcohol containing 1 to 4 carbon 
atoms, the pressure and temperature during the transesterification being 
selected such that the higher alcohol employed and the silanes do not 
boil. 
DESCRIPTION OF THE INVENTION 
Within the scope of the present invention, the term "silanes" is intended 
to be understood also as mixtures of silanes and partial hydrolysis 
products thereof containing at most 5 silicon atoms, such partial 
hydrolysis products preferably being present only in small quantities such 
as, for example, up to 10 percent by weight, relative to the silane, or 
not at all. 
The alkoxysilanes which are employed according to the invention and whose 
alkoxy groups contain 1 to 4 carbon atoms, can be any hitherto known 
alkoxysilanes, such as silanes having 1, 2, 3 or 4 Si-bound alkoxy groups, 
tetraalkoxysilanes having alkoxy groups containing 1 to 4 carbon atoms 
being preferred and tetramethoxysilane as well as tetraethoxysilane being 
particularly preferred. 
The alkoxysilanes employed according to the invention can be a single 
species or also a mixture of at least two species of such alkoxysilanes. 
The alcohols employed according to the invention are linear and/or branched 
alcohols which contain 6 to 38 carbon atoms and which can be aliphatically 
saturated or unsaturated. 
Preferably, the alcohols employed according to the invention are 
aliphatically saturated alcohols containing 8 to 20 carbon atoms, such as 
1-n-octanol, 1-n-decanol, 1-n-dodecanol, 1-n-tetradecanol and mixtures of 
linear and branched alcohols containing 13 to 15 carbon atoms, such as, 
for example, those commercially available under the designation "C13-C 
15-alcohol" from BASF AG, Ludwigshafen. 
The higher alcohols employed according to the invention can be a single 
species or also a mixture of at least two species of such alcohols. 
In the process according to the invention, alkoxysilanes whose alkoxy 
groups contain 1 to 4 carbon atoms are employed together with higher 
alcohols containing 6 to 38 carbon atoms in molar quantities of preferably 
1:4 to 1:4.5, particularly preferably about 1:4. 
In the process according to the invention, basic and acid catalysts can be 
employed as the catalysts, basic catalysts being preferred. 
Examples of basic catalysts are homogeneous catalysts such as, for example, 
alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, 
and alkali metal alcoholates such as sodium methylate, sodium methylate 
being preferred. 
Examples of acid catalysts are homogenous catalysts such as hydrochloric 
acid, sulphuric acid and p-toluene-sulphonic acid, as well as 
heterogeneous catalysts such as solid acid cation exchangers and solid 
acid-activated bleaching earths, such as, for example, those of the 
montmorillonite type. 
The homogeneous catalysts are used in quantities of preferably 0.01 to 1.5 
percent by weight, particularly preferably from 0.05 to 1 percent by 
weight, and the heterogeneous catalysts are preferably used in quantities 
of 1 to 20 percent by weight, particularly preferably from 5 to 15 percent 
by weight, relative to the employed alkoxysilane having alkoxy radicals 
containing 1 to 4 carbon atoms. 
The process according to the invention is carried out at a temperature of 
preferably 30 to 140.degree. C. 
The process according to the invention is carried out under a pressure of 
preferably 5 to 300 hPa. 
In the transesterification according to the invention, the pressure and 
temperature are to be selected such that the resulting alcohol containing 
1 to 4 carbon atoms can be removed by distillation and the higher alcohol 
employed and the silanes do not boil. It can prove to be advantageous 
towards the end of the transesterification according to the invention to 
reduce the pressure again briefly, that is to say to lower it below the 
pressure selected during the tranesterification, in order completely to 
remove residual fractions of the alcohol formed during the 
tranesterification. 
If homogeneous acid and basic catalysts are employed as catalysts in the 
process according to the invention, these are preferably neutralized after 
the tranesterification has taken place, and the salt formed is preferably 
removed by filtration. The neutralization as such has been known for a 
long time, it being of course possible to use the same acid and basic 
compounds which have been listed above also as catalysts in each case. 
If heterogeneous catalysts are employed as catalysts in the process 
according to the invention, these are removed from the reaction mass after 
the transesterification has taken place--for example by filtration. 
The products obtained in the process according to the invention are 
light-coloured, preferably colourless alkoxysilanes having alkoxy radicals 
containing 6 to 38 carbon atoms. 
Using the process according to the invention, a degree of 
transesterification of from preferably 95 to 100%, particularly preferably 
98 to 100%, is achieved. 
The process according to the invention has the advantage that it is very 
simple to carry out, light-coloured, preferably colourless products being 
formed at a very high degree of transesterification, preferably 100%, in 
one process step within a relatively short time and without a further 
purification step. 
In the examples which follow, all data of parts and percentages relate to 
the weight, unless otherwise stated. Unless indicated otherwise, the 
examples which follow are carried out under a pressure of the surrounding 
atmosphere, that is to say approximately under 1000 hPa, and at room 
temperature, that is to say at about 20.degree. C. or at a temperature 
which is established when the reactants are combined at room temperature 
without additional heating or cooling. All viscosity data given in the 
examples are intended to relate to a temperature of 25.degree. C. 
The iodine colour number is determined according to DIN (Deutsche Industrie 
Norm--German Industiral Standard) 6162. The iodine colour number is a 
measure of the degree of discoloration of a product and is the quantity of 
iodine in mg per 100 ml of an aqueous solution whose depth of colour 
corresponds to that of the product to be examined in the same layer 
thickness.

EXAMPLE 1 
179 g (0.96 mol) of 1-n-dodecanol (commercially available from 
Merck-Schuchardt, Hohenbrunn), 50 g (0.24 mol) of tetraethoxysilane 
(commercially available under the designation "TES 28" from Wacker-Chemie 
GmbH, Munich) and 0.5 g of a 30% solution of sodium methylate in methanol 
are mixed. Subsequently, the reaction mixture is heated up within 120 
minutes under a pressure of 160 hPa to 130.degree. C. and ethyl alcohol, 
being formed at the same time, is distilled off. The pressure is then 
lowered briefly to 40 hPa in order to remove residual ethyl alcohol 
fractions, the mixture is neutralized by the addition of 0.5 g of 20% 
hydrochloric acid and the resulting sodium chloride is filtered off. This 
gives a clear, colourless (iodine colour number=0) liquid having a 
viscosity of 27.5 mm.sup.2 /s and an HCl content of 2.5 ppm. According to 
.sup.1 H-NMR investigations, the product no longer shows any Si-ethoxy 
groupings nor any free ethyl alcohol. 
Comparative Example 1 
The procedure of Example 1 is repeated, but with the modification that the 
ethyl alcohol being formed during the reaction is distilled off under the 
pressure of the surrounding atmosphere, that is to say at about 1000 hPa. 
For this purpose, the reaction mixture must be heated up to 170.degree. C. 
within 6 hours. The mixture is then in neutralized by the addition of 0.5 
g of 20% hydrochloric acid and the resulting sodium chloride is filtered 
off. 
This gives a clear liquid having a viscosity of 20.7 MM.sup.2 /s, an HCl 
content of 0.5 ppm and an iodine colour number of 2. According to .sup.1 
H-NMR investigations, the product still contains 13.2 mol % of Si-ethoxy 
groupings and 4.4 mol % of ethanol. 
EXAMPLE 2 
The procedure of Example 1 is repeated, but with the modification that, in 
place of 179 g of 1-dodecanol, 205.9 g (0.96 mol) of a mixture comprising 
linear and branched primary C13-C15-alcohols (commercially available under 
the designation "C13-C15-alcohol" from BASF AG, Ludwigshafen) are 
employed. 
This gives a clear colourless (iodine colour number=0) liquid having a 
viscosity of 37.3 mm.sup.2 /s and an HCl content of 3 ppm. According to 
.sup.1 H-NMR investigations, the product no longer shows any Si-ethoxy 
groupings nor any free ethyl alcohol. 
Comparative Example 2 
The procedure of Example 2 is repeated, but with the modification that 
ethyl alcohol being formed during the reaction is distilled off under the 
pressure of the surrounding atmosphere, that is to say at about 1000 hPa. 
For this purpose, the reaction mixture must be heated up to 160.degree. C. 
within 6 hours. The pressure is then briefly lowered to 40 hPa in order to 
remove residual fractions of ethyl alcohol, the mixture is neutralized by 
the addition of 0.5 g of 20% hydrochloric acid and the resulting sodium 
chloride is filtered off. 
This gives a clear liquid having a viscosity of 36.1 mm.sup.2 /s, an HCl 
content of 3 ppm and an iodine colour number of 1. According to .sup.1 
H-NMR investigations, the product still contains about 0.2 mol % of 
Si-ethoxy groupings.