Note: Descriptions are shown in the official language in which they were submitted.
o. æ . 4497
HULS AKTIENGESELLSCHAFT
- PA.TENT DEPARTMENT -
P cess for the preparation of di-tert.-butoxydiacetoxy-
silane
The present invention relates to a process for the
preparation of di-tert.-butoxydiacetoxysilane from
tetraacetoxysilane and tert.-butanol.
This silicon compound is suitable, in particular, as a
crosslinking agent in the preparation of compositions
which have a long shelf life in the absence of water and
are curable at room temperature on contact with moisture
to form elastomers. Such compositions are obtained by
mixing dioryanopolysiloxanes containing condensation-
capable end groups with crosslinking silicon compounds.
It is known that di-tert.-butoxydiacetoxysilane can be
prepared by reacting di-tert.-butoxydichlorosilane with
acetic acid in the presence of suitable acid acceptors
and solvents (US Patent 2,566,957). It is disadvantageous
in this process that, depending on the acid acceptor
employed, amine hydrochlorides are produced in a form
which is very finely divided, difficult to filter and
difficult to wash out. The yields whlch can be achieved
with the proc:edure u~ed are about 76~ and require the u~e
of the starting material dl-tert.-butoxydichlorosilane,
which is only accessible by a complex process.
It is furthermore known that alkoxyacetoxysilane~ can be
prepared by reacting alcohols with tetraacetoxysilane
(Zhurnal obsce~ Chimii 27 (1957~, p. 921 to 926). In this
publication, it is stated that tertiary alcohols react
with tetraacetoxysilane only with difficulty. If tert.-
butanol is used as the alcohol component, the reaction
mixtures must be warmed to temperatures of from 100 to
140C in order to ob1:ain tert.-butoxyacetoxysilanes.
Thus, to prepare di-tert.-butoxydiacetoxysilane, it was
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necessary to heat a mixture of tetraacetoxysilane and tert.-
butanol in the molar ratio 1:2 at up to 100C for 4 hours. Even
then, the product yield obtained from the reaction mixture after
distillative work-up was only 48%.
The object was therefore to prepare di-tert.-butoxydi-
acetoxysilane in a process in which no starting materials which
can only be produced by means of considerable technical complexity
are used and in which the starting materials employed are con-
verted into the taryet product as quantitatively as possible.
Thus the present invention provides a process for the
preparat:ion of di-tert.-butoxydiacetoxysilane, which process
comprises reacting te-traacetoxysilane and tert.-butanol at a
temperature of up to about 80C and isolating the di-tert.-butoxy-
diacetoxysilane so produced.
A preferred embodiment of the process comprises carrying
out the reaction of the components tetraacetoxysilane and tert.-
butanol in the prescnce of the reaction mixture as produced during
the preparation of tetraacetoxysilane from sil:icon tetrachloride
and acetic acid.
The sequence in which the -rcac-tants act on one another
is imrna-terial for the course of the reaction. The preferred form
of the react:ion course comprises allowiny the tert.-butanol in
liqu:id Eorm to act on thc initially introduced solid tetraacetoxy-
silane. Thc end of the rcaction can be detectcd rom the initially
heterogcncous systern becoming homogeneous. This state is
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established about 30 minutes after completion of the combination
of the reactants. In contrast to the process described in the
abovementioned publi.cation (Zhurnal obscej Chimii), the reaction,
which is known per se, proceeds according to the invention in
the claimed temperature range within a short time.
Subsequen-t heating after a homogeneous reaction solution
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has been prcd~ced is not necessary.
The reacti~n cf tetraacetcxy~ila~e ~ith tert.-butanol can
also be carried out in the presence of inert media.
Substances of this type, such as aliphatic hydrocarbons,
aromatics or chlorLnated hy~rocarbons, have no specific
effect on tk~e reaction proce~dings.
The use of aliphatic hy~rccarbo~s as solvent is par-
ticularly a~Lsable if the starting material tetra-
acetoxysil2~-e is the ~n-wcrk2d-up product of the reaction
of SiCl~ an~ acetic acid.
Since the re c~ion is not accompanied by any evolution of
heat, it is possible to c~m~ine the reaction components
tetraaceto~ysilane and tert.-butanol without problems,
even in relztiYely large æmounts.
The reactic~ can be carr-e~ o~tt elther continuously or
batchwise; the reaction is ~referably carried out batch-
wise. In a cor.~inuous p~cce~ure, the alcohol component
and the tet-aacetorysila~e, if desired suspended in an
inert mediu~, are fed sim~lt~neously, taking into account
the stoick~ometric circum~tances, into a 3uitable
reactor, fo~ e~ample a t~iular reactor. After an appro-
l priate re~ e~ce tL~e, ~he reaction procluct di-tert.-
butoxydiacetory~i1~n~, ~ u~s of unreacted starting
materials, poss~bly saal~ 2~mounts of byproducts and the
acetic acid als~ forred lea~e the reactor.
The reactiol o~ the reaction components tetraacetoxy-
silane and rertA-butanol ~g carrie~ out according to the
invention a~ t~mperatures of up to 80C. When these
reaction te~peratures are used, improved yields of di-
tert.-butosydL2ceto~ysila~e are obt~ined, which are up to
100% above he yields for ~e prep~ration methods known
hitherto.
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In the reaction of the reaction components tetraacetoxy-
silane and tert.-butanol at the claimed temperatures of
up to ~0C, the resultant yields of di-tert.-butoxydi-
aceto~ysilane vary only insignificantly, a sliqht ten-
S dency toward a reduction in yield being detectable withincreasing reaction temperature. For this re~son, the
reaction is preferably carried out in the temperature
range from 40 to 70 C. If reaction temperatures above
80C are used, the abovementioned reductions in the yield
of di-tert.-butoxydiacetoxysilane occur. The lower limit
of the temperature range is determined by the melting
point of the tert.-butanol, which can be lowered to
temperatures of about 0C by adding suitable solvents. It
is thus important that the tert.-butanol is in the form
of a liquid phase during the reaction.
The reaction components tetraacetoxysilane and tert.-
butanol are preferahly employed in a molar ratio of from
1:1.9 to 2.2.
It has proven expedient to react the reaction components
in the molar ratio 1~2. If this molar ratio is chosen,
reaction proclucts are obtained in the claimed temperature
range which are essentially free from the byproducts
tert.-butoxytriacetoxysilaneandtri-tert.-butoxyacetoxy-
I silane.
The reaction i8 carried out at atmospheric pressure. The
use of reduced pressure or superatmospheric pres~ure is
possible, but has no significant effect on the reaction
proceedings. The cxude products obtained using the
process according to the invention are worked up in a
manner which is known per se. Any solvent used and the
acetic acid formed during the reaction are preferably
removed by distillation. Irrespective of the composition
of the crude product, the work-up i8 carried out in vacuo
from the outset. Precautions are taken during the entire
work-up process to ensure that the phase containing the
reaction product di-tert.-butoxydiacetoxysilane is not
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warmed to above 90C for any length of time.
Af~er removal of all the low-boiling substances from the
reaction mixture by distillation, di-tert.-butoxydi-
acetoxysilane is obtained in a quality which is adequate
for many appli~ations. Use in demanding areas of applica-
tion requires a particular product quality, which is
obtained by final distillation in a thin-film evaporator
in vacuo at temperatures of below ~0C.
The invention is illustrated with reference to the
examples given below:
Example 1
1056 g (4 mol) of tetraacetoxysilane are introduced into
a 4 litre twin-jacket flask equipped with a reflux
condenser, stirrer, dropping funnel and thermometer and
heated by means of a temperature-controlled heating
circuit. 480 g (8 mol) of tert.-butanol are added from
the dropping funnel over the course of 3 minutes to the
initially introduced tetraacetoxysi]ane, which is kept at
a temperature of 23C via the temperature-controlled
heating circuit. The reaction is complete within 30
minutes, which can be d0tected from the complete di~ap-
pearance of the tetraacetoxy~ilane. ~n~ly~i~ of the flask
product by gals chromatography (GC) indicates the prHSenCe
of the targe1: product di-tert.-butoxydiacetoxysilane in
addition to acetic acid. The crude product is transferred
into a distillation apparatus, which comprises a twin-
~acket distillation still with connected temperature-
controlled heating circuit and inserted thermometer, and
a distillation attachment which i8 connected to a vacuum
pump via a cold trap. The crude product employed is freed
from all the acetic acid within two hours at a bottom
temperature of up to a maximum of 85C. A c].ear, vir-
tually colourless liquid (1153 g) which has a purity (GC)
of 97~ rem~ins. Distillative work-up of the still product
in a glass laboratory thin-film evaporator in vacuo gives
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a distillate of 1131 g having a purity (GC~ of 98% and a
residue of 22 g.
Example 2
The experiment of Example 1 is repeated, but the reaction
temperature used is 80C.
Analysis of the crude reaction product by GC indicates
the presence of the target product in addition to acetic
acid, extremely small amounts of the compounds tert.-
butoxytriacetoxysilane,tri-tert.-butoxyacetoxysilaneand
~iloxanes.
After removal of the acetic acid by distillation, a
slightly yellowish still residue (1152 g) having a purity
(GC) of 93.2~ remains. Work-up of this still residue in
a thin-film evaporator gives a distillate of 1109 g
having a purity of 96.8~ and a residue of 40 g.
Example 3 (comparative exam~le)
The experiment of Example 1 is repeated, but the reaction
temperature used is 115C.
( Analysis of the crude reac~ion product by GC indLcates
the presence of the target product in ~ddition to acetic
acid. In addition, an increased presence of siloxane6 is
determined. After removal of the low-boiling components
by distillation, a yellow residue (1096 g) containing
43.7~ of di-tert.-butoxydiacetoxysilane remains in the
distillation still. Distillative work-up gives 465 g of
distillate and 598 g of residue.
Example 4
The experiment of Example 2 is repeated, but with 2000 ml
of octane added to the initially introduced amount of
tetraacetoxysilane. After removal of the octane and the
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acetic acid by distillation, a slightly yellowi~h re~idue
(1160 g) having a purity of 93.9~ remains in the distil-
lation still. Work~up in a thin-film evaporator gives a
distillate of 1119 g having a purity of 97.3%, and 38 g
of residue.
Example 5 (comparative example)
The experiment of Example 3 is repeated, but with 2000 ml
of octane added to the initially introduced amount of
tetraacetoxysilane.
After removal of the octane, the acetic acid and low-
boiling components by distillation, a yellow product
(1096 g) containing 59.7% of di-tert.-butoxydiacetoxy-
silane (GC) remains in the distillation still.
Distillative work-up of this still residue gives 658 g of
distillate and 434 g of residue.
Example 6
500 kg of silicon tetrachloride and 400 litres of hexane
are introduce!d into a 3 m3 reactor fitted with a cooling
attachment and connected via the latter to a water-pump
vacuum. 740 ]cg of acetic acid are matered ln over the
cour~e of 5 hours at a reaction medi.um temperature of
70C. The hydrogen chloride formed continuously i~
removed by ~uction. When it has been removed completely,
436 kg of tert.-butanol are added to the tetraacetoxy-
silane/hex~ne system over the course of 20 minutes. 30minutes later, the reactor contents are tran~ferred into
a distlllation apparatus, and the low-boiling components
(hexane, acetic acid and others) are removed in vacuo.
The pale-yellow still residue (856 kg, purity (GC) 93.6%)
is, after filtration, either packaged as a commercial
product or immediately worked up in a thin-film eva-
porator. In the latter case, 825 kg of distillate having
a purity (GC) of 97~ are obtained.
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_ample 7
1050 g of acetic anhydride are in~.oduced into a reaction
flask fitted with a distillation attachment. 350 g of
silicon tetrachloride are metered in over the course of
about 5 minutes at 25C. The precipitated tetraacetoxy-
silane is freed from acetyl chloride formed and excess
acetic anhydride by distillation. 350 q of tert.-butanol
are added to the dry tetraacetoxysilane material at room
temperature. The di-tert.-butoxydiacetoxysilane/acetic
acid mixture produced is freed from acetic acid by vacuum
distillation and subsequently purified by distillation in
a thin-film evaporator. 578 g of colourless di-tert.-
butoxydiacetoxy~ilane having a purity (GC) of 98~ are
obtalned in this way.
