Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
2002-06-06
2003-06-17
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
Reexamination Certificate
active
06580000
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to the production of alkoxysilanes and alkoxy orthosilicates by reacting silicon metal with alcohols in the presence of a suitable catalyst.
Alkoxysilanes are useful chemicals for the synthesis of organosilanes used as silane-coupling reagents. Trimethoxysilane (HSi(OCH
3
3)
3
3), for example, can be synthesized directly by reaction of silicon metal with methanol in the presence of copper(I) chloride as a catalyst. Pioneering work by E. Rochow built the direct reaction of silicon with alkyl chlorides into the silicone industry. Rochow discovered that a mixture of elemental copper in silicon provided optimum conditions for reactions in gas fluidized reactors. Later work by the Japanese involved liquid phase reactions using copper chloride catalyst in an aromatic solvent at high temperatures.
Depending upon the reaction conditions, pretreatment conditions, and the particular catalyst chosen, the yield of the desired trialkoxysilane can vary widely. Tetraalkyl orthosilicate is a common by-product (and also a valuable by-product with sufficient commercial value), formed either directly from the reaction of elemental silicon and alcohol, or from the secondary reaction of trialkoxysilane and alcohol. Depending upon the identity of the particular alcohol used in the reaction, alcohol reduction, dehydration and/or dehydrogenation side reactions also may be problematic. Where triethoxysilane is the desired end-product, thermal degradation of the ethanol reactant can lead to olefins and their incorporation into the silane reaction.
U.S. Pat. No. 5,728,858 discloses a direct process for producing trialkoxysilanes in which silicon metal is slurried in a thermally stable solvent in the presence of a halogen-free catalyst precursor. The catalyst precursor includes copper, at least a part of which is not in the copper(0) state and is reducible to the copper(0) state. The copper(0) is then fully reduced to generate a catalyst for the reaction of the silicon metal with an alcohol, and the reaction is carried out. Suitable thermally stable solvents disclosed include polyaromatic and alkylated aromatic compounds. However, previous documents claim aromatics are critical but do not clearly describe chemistry of their role in their process. Using simpler chemical structures can avoid problems in process improvement, control and environmental management.
It therefore would be desirable to provide a process for the production of alkoxysilanes and or alkyl orthosilicates from silicon metal and alcohol in high yield that does not suffer from the drawbacks of the prior art.
It further would be desirable to provide a process for the production of alkoxysilanes and/or alkyl orthosilicates from silicon metal and alcohol that eliminates the pre-reduction of copper to form the catalyst.
It further would be desirable to provide a process for the production of alkoxysilanes and/or alkyl orthosilicates using a liquid phase reaction without an exotic solvent and into which a catalyst could be introduced in order to control the reaction rate.
It still further would be desirable to provide an improved catalyst for the reaction of silicon metal with alcohol to produce alkoxysilanes and/or alkyl orthosilicates in high yield.
Other objects and advantages of the present invention will be made apparent by the following description and examples.
SUMMARY OF THE INVENTION
The problems of the prior art have been overcome by the present invention, which provides a process for the production of alkoxysilanes of the formula HSi(OR)
3
, where R is an alkyl group containing 1 to 6 carbon atoms, and/or alkyl orthosilicates of the formula Si(OR)
4
, where R is an alkyl group containing 1 to 6 carbon atoms. In a preferred embodiment, the process comprises a slurry reaction wherein silicon metal is reacted with an alcohol in a suitable solvent in the presence of a cupric bis(diorganophosphate) catalyst. A polymeric form of ethyl orthosilicate, a by-product of the reaction, is the preferred solvent. The production of triethoxysilane and tetraethyl orthosilicate is preferred, with triethoxysilane being particularly preferred. Auxiliary reduction with hydrogen or other reducing agents is not required. Judicious addition of catalyst and solvent during the reaction can sustain reaction rate and selectivity.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention is based upon the following chemical reactions:
Si+3ROH+catalyst →HSi(OR)
3
+H
2
wherein R is an alkyl group of 1 to 6 carbon atoms, preferably 1 to 2 carbon atoms, most preferably 2 carbon atoms. The major by-product of this reaction forms the tetraalkyl orthosilicate:
Si+4ROH+catalyst →Si(OR)
4+
2H
2
In addition, the trialkoxysilane can further react in the presence of sodium hydroxide as follows:
HSi(OR)
3
+H
2
O →(EtO)
3
SiOH+H
2
The process conditions and catalyst of the present invention favor the production of trialkoxysilane.
In accordance with the present invention, the liquid reaction medium is an organosilicate, preferably alkyl orthosilicate or alkyl orthosilicate polymer. Most preferably the alkyl group is ethyl. Advantageously, alkyl silicate or derivatives thereof are by-products of the reaction to trialkoxysilane. Suitable derivatives are of the formula Et(OSi(OEt)
2
)
n
OEt, where n is from 1 to 10, preferably 2 to 5. The most preferred medium is a condensation polymer of tetraethyl orthosilicate corresponding analytically to the pentamer as discussed in further detail below. Tetraethyl orthosilicate itself also can be used. The reaction medium can periodically cleaned such as by extraction and distillation.
More specifically, the basic composition unit of the solvent is tetraethyl orthosilicate or TEOS ((EtO)
4
Si) commercially known as “Ethyl Silicate Pure” when distilled, or “Ethyl Silicate Condensed” when obtained without further purification from the manufacturing process. This composition contains 28% SiO
2
. Ethyl Silicate-40 or “ES-40” is the major commercial form of ethyl silicate, and contains 40% SiO
2
. It is easily made from “ethyl Silicate Condensed” by adding water and a small amount of hydrochloric acid, then distilling the appropriate amount of ethanol. This reaction not only forms dimers and trimers, but a complex permutation of structures in three dimensions. Data demonstrate that the dimer boils at 235°, whereas the “monomer” TEOS boils at 170° C. Higher molecular weight polymers of TEOS boil above 250°. Thus, the dimer and higher “condensation polymers” of TEOS are well suited for the instant process since they will not boil off with the products TES and TEOS.
By SiO
2
content, ES-40 is calculated to average 5 units of monomer, but by analysis commercial ES-40 contains about 25% TEOS. The remaining 75% is polymer ranging from dimer to diverse three dimensional structures containing up to a dozen TEOS units. The less volatile portion of ES-40 ideally remains in the reactor and provides a suitable medium for reacting silicon with ethanol in the presence of catalyst.
The higher molecular weight polymers of TEOS can gel upon prolonged heating and reaction with impurities (especially water) in reactants. ES-40 holds up without significant gelation when held below 220° C. for 24 hours, which is sufficient to process a charge of silicon, after which the solvent medium can be re-worked by removing un-reacted silicon and less soluble components formed in the reaction medium. The re-worked medium then can be recycled to the next batch.
Thus the reaction medium can be the dimer, the trimer, higher polymeric organosilicates formed by the reaction of TEOS and TES by water and heating, or mixtures thereof.
The elemental silicon used as a reactant in the process of the present invention is not particularly limited. Suitable sources include commercially available grades of silicon, in particulate or powder form. Purities of commercial grade silicon are in the range of about 80% to about 99% by weight, with particle sized
Anderson Amos R.
Meyer Jeffrey G.
AK Research Company
Nields & Lemack
Shaver Paul F.
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