Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
1999-02-05
2001-02-20
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
Reexamination Certificate
active
06191296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a continuous method of manufacturing chlorine-free tetraalkoxysilanes from metallic silicon powder and alcohol in the presence of a liquid, basic catalyst mixture with improved selectivity and improved space-time yield.
2. Description of the Background
Methods of continuous production of tetraalkoxysilanes are known, and essentially two methods are used in industry: The first method is the direct reaction of silicon powder with alcohol, while the second method is the esterification of tetrachlorosilane with alcohol.
The latter method, esterification of tetrachlorosilane with alcohol, can be conducted at normal pressure and without energy input in simple reactors. The ester mixture which is produced must be distilled, however, in order to remove, in particular, chlorine-containing impurities such as chlorine-containing esters and dissolved hydrogen chloride. The cost of such distillation is very high if one wishes to obtain a product tetraalkoxysilane of low chlorine content. For example, such distillation is conducted in a column having a large number of plates, in countercurrent distillation with alcohol. The residual chlorine in the tetraalkoxysilane product is neutralized in an additional reaction stage. The salt thereby obtained must be separated and appropriately discarded, and the tetraalkoxysilane must undergo additional distillative purification. Further, the esterification of tetrachlorosilane with alcohol involves the liberation of gaseous hydrogen chloride, which is costly to purify if it is to be economically recovered. The method essentially requires two reaction stages to convert the silicon to tetraalkoxysilane, viz. synthesis of the chlorosilane, and esterification of the chlorosilane. For more efficiency there is a need for a method employing only one reaction stage.
A method of producing alkoxysilanes in a single step reaction is the direct synthesis from silicon and alcohols. In U.S. Pat. No. 5,084,590, such a direct synthesis in the presence of a copper catalyst is described. However, the predominant product of this reaction is the trialkoxysilane. Where the objective of the direct synthesis is tetraalkoxysilanes, there is no known way of optimizing the reaction conditions which appreciably avoids formation of Si—H bonds and thereby formation of trialkoxysilanes. Accordingly, yields of tetraalkoxysilanes are unsatisfactory. Moreover, costly processing of the copper-containing residues is required, rendering the copper-catalyzed synthesis unsuitable for economical and efficient manufacture of tetraalkoxysilanes.
German 28 16 386 describes a catalytic method of manufacturing tetraalkoxysilanes from silicon and alcohols without the introduction of heavy metals such as copper. The catalysts comprise alkali alcoholate, ether alcohols and alkanolamines. Preferably, the method is conducted in a stirred vessel. The yield does not exceed 90 wt. %, based on the silicon introduced, which is not sufficiently high for an efficient continuous process. A side reaction of hydrogen with the alcohols occurs, forming water and alkanes, which necessitates a costly distillation with a scavenger, e.g., azeotropic distillation, in order to remove the water. The water present deactivates the catalyst and also leads to undesirable oligomerization of the tetraalkoxysilanes.
U.S. Pat. No. 4,288,604 describes a discontinuous method of reacting silicon and alcohols to form tetraalkoxysilanes in the presence of a basic catalyst. The silicon used must be comminuted under a nitrogen atmosphere in order to avoid blocking of the silicon surface with oxygen. In order to enable initiation of the reaction, inert, high-boiling solvents are employed. In addition, hydroxide scavengers must be employed in order to deactivate hydroxyl groups which are formed or which are introduced with reaction mixture components. Despite the use of appreciable amounts of inert solvents, hydroxide scavengers, and alkali alcoholates (as catalysts), the method is unsuitable for a continuous process, because it falls far short of the necessary high conversions of the silicon and ethanol to product. A need, therefore, continues to exist for a continuous process of producing tetraalkoxysilanes in high yield.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a simple, continuous method of manufacturing chlorine-free tetraalkoxysilanes, which method enables direct reaction of metallic silicon powder and alcohols with improved selectivity and space-time yield of product.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a continuous method of manufacturing chlorine-free tetraalkoxysilanes of formula (I)
(R
1
O)
4
Si (I)
wherein R
1
is, independently in each instance, an unbranched, branched, and/or cyclic C
1-6
hydrocarbon group, comprising:
reacting metallic silicon powder with a monohydric alcohol in the presence of a basic catalyst comprising a mixture of:
(a) at least one compound of formula (II)
[R
2
O—(R
3
O)
m
]
n
Si(OR
1
)
4-n
(II)
(b) at least one compound of formula (III)
R
2
O—(R
3
O)
m
—M, and (III)
(c) at least one compound of formula (IV)
R
2
O—(R
3
O)
m
—H (IV)
wherein R
1
is as defined above;
R
2
represents, independently in each instance, a C
1-10
hydrocarbon group, or hydrogen;
R
3
represents, independently in each instance, a C
1-4
hydrocarbon group;
M represents an alkali metal selected from the group consisting of Li, Na, and K;
m is zero or a number in the range of 1-20; and
n is zero or a number in the range of 1-4; further comprising:
charging said catalyst in advance to a continuously operating reactor to which at elevated temperature is also continuously added metallic silicon powder and the alcohol; and then
separating the reaction mixture in a first column, in which gaseous hydrogen is discharged from the top, high-boiling catalyst is returned to the reaction mixture in said reactor, and low-boiling components are sent to a second separation column in which the tetraalkoxysilanes of formula (I) are separated, and low-boiling alcohol(s) present is/are turned to the reactor.
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patent: 4288604 (1981-09-01), Magee et al.
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patent: 5103034 (1992-04-01), Cho et al.
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patent: 5728858 (1998-03-01), Lewis et al.
patent: 5783720 (1998-07-01), Mendicino et al.
patent: 0 127 951 (1984-12-01), None
patent: 0 225 137 (1987-06-01), None
Dittrich Uwe
Mueller Reinhard
Steding Frank
Huls Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shaver Paul F.
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