Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Reexamination Certificate
2000-09-25
2002-11-19
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C423S342000, C556S474000
Reexamination Certificate
active
06482381
ABSTRACT:
The invention relates to a process for the direct hydrogenation of halogen-substituted silicon compounds. Hydrogen-substituted silicon compounds have achieved particular industrial importance. Thus, monosilane (SiH
4
) is used for producing high-purity silicon for semiconductor technology and, like disilane (Si
2
H
6
), for the (epitaxial) deposition of thin layers in microelectronics in the production of chips and thin-film solar cells. Moreover, in the preparation of organofunctional (poly)siloxanes, hydrogen-substituted organochlorosilanes, for example (CH
3
)
2
SiHCl, (CH
3
)SiH
2
Cl, (CH
3
)SiHCl
2
, etc., which are obtained only in insignificant amounts or in amounts which do not correspond to the respective demand in the industrial Müller-Rochow direct synthesis, are key compounds for the known hydrosilylation reaction.
There has therefore been no lack of attempts to produce the various silanes on an industrial scale. Neither the acidolysis of silicides nor the reaction of silicon tetrachloride with lithium alanate in ether solution are suitable for producing silanes inexpensively and in the required purity. The catalytic disproportionation of trichlorosilane carried out on a relatively large scale forms not only one part of monosilane but also three parts of silicon tetrachloride as by-product which has to be worked up. Finally, alanates or amine-alane adducts (R
3
N—AlH
3
) prepared therefrom are reacted with silicon tetrafluoride in organic solvents to form monosilane.
A solvent melt as reaction medium has been used in a continuous or cyclic process and developed for industrial use. In this process, silicon tetrachloride is hydrogenated by means of lithium hydride which is prepared from lithium in an LiCl-KCl melt at 400° C., dissolved in the process and subsequently reacted. Disadvantages are the high temperature, corrosion problems, the high price of lithium compounds and the environmental concerns associated with them, and finally the reprocessing cost for the lithium chloride formed [(1) DE-C 1080077; (2) W. Sundermeyer et al. Angew. Chem 70 (1958) 625].
The direct hydrogenation of silicon tetrachloride in a eutectic mixture of sodium chloride and aluminum chloride (m.p. 108° C.) is described by H. L. Jackson et al. [Inorg. Chem. 2 (1963) 43]. In the reaction of hydrogen with a high molar excess of aluminum powder, 30 g of silicon tetrachloride are hydrogenated at 175° and an industrially impractical pressure of 960 bar in a 400 ml shaking autoclave charged with steel balls for 6 hours to give a conversion of 70-100% and 5.8 g of monosilane. Disilane and dimethylsilane are formed only in very small amounts under analogous reaction conditions because of cleavage of the Si—Si or Si—C bonds. Monosilane is also formed only in traces when the aluminum is not additionally treated with up to 3% of an “activator” such as lithium hydride, lithium alanate, sodium hydride, alkaline earth metal hydrides and the abovementioned melt. The reaction mechanism is said to be the formation of H
x
AlCl
3−x
as hydrogenating agent.
The analogous reaction of (organo)chlorosilanes with hydrogen and aluminum in salt melts comprising sodium chloride and aluminum chloride is described by H. J. Klockner et al. [EP 0412342] as proceeding under atmospheric pressure when the aluminum contains 0.03-0.25% by weight of a hydrogen-transferring metal such as titanium, zirconium, hafnium, vanadium, niobium or nickel, according to the examples as alloy. In addition, this document also claims, inter alia, the same “activators” as those described by H. L. Jackson et al.: Lithium alanate and sodium hydride, and additionally titanium hydride. In another document [DE 4119578.7], lithium hydride (cf. H. L. Jackson et al.) and also zirconium hydride, palladium chloride, nickel chloride and Raney nickel are claimed as “activators”. As regards the reaction mechanism, the generation of H
x
AlCl
3−x
as actual hydrogenating compound which is said to be present in a concentration of 0.01-20 mol %, in particular 10 mol %, based on the amount of melt, is once again described. When using a melt having a molar ratio of NaCl:AlCl
3
=1:1, 90% yields of SiH
4
and 83-90% yields of (CH
3
)
2
SiH
2
were observed. However, the conversions (% of the total starting compound used which is reacted) and, even more, the space-time yields (STY; g/l of reaction volume and hour) of the individual components are extraordinarily low. In the case of SiCl
4
, the conversion is 20% (66 g of 324 g in 6 h) and the STY is only 1.9 g of SiH
4
/lh. (CH
3
)
2
SiH
2
was able to be obtained in a conversion of only 5-8% (15.5 g of 327 g in 2 h) and an STY of 3-6 g/l h. In each case, only up to about 3% of the hydrogen introduced was reacted, since according to the method described it has to be used in a large, 5-10-fold excess.
It is therefore an object of the invention to develop a process for the direct hydrogenation of Si—Cl bonds using elemental hydrogen at moderate temperatures and pressures, which process avoids the stated disadvantages and makes it possible to prepare silanes in a technically simple and economical manner.
Surprisingly, metals which form interstitial hydrides, preferably titanium, zirconium, vanadium, chromium, manganese, nickel, palladium, platinum and rare earth metals, or combinations thereof, in particular titanium metal, have now been found to be particularly good hydrogen transferrers for achieving this object when they are present in suspension in very finely divided form, preferably when they are produced in situ in a salt melt comprising alkali metal/alkaline earth metal halides and aluminum halide, in particular the chlorides, by reduction of the corresponding metal halide by an electropositive element (halogen acceptor I). As metal halides, it is possible to use all solid or volatile compounds, for example in the case of titanium the compounds of the type TiX
4
, TiX
3
, TiX
2
, or salts containing TiX
6
2−
or TiX
6
3−
ions (X=halogen), but preferably TiCl
4
or TiCI
3
, which are dissolved in the melt and reduced to titanium metal by the halogen acceptors I, namely magnesium, calcium or aluminum. The so called “activators” in the form of various previously prepared, expensive hydrides or alloys as are known from the prior art are not necessary in the process of the invention.
The salt melt serving as reaction medium comprises mixtures of alkali metal halides and/or alkaline earth metal halides and aluminum halides, preferably the chlorides. The liquidus curves of the individual systems can be found in standard tables [R. S. Roth, M. A. Clevinger, D. McKenna, Phase Diagrams for Ceramists, National Bureau of Standards, The American Ceramic Society, Inc., Vol. I-V, 1964-83]. The eutectics or melts even richer in AlCl
3
can be used, but they suffer from a high degree of AlC
3
sublimation, which would require appropriate, technically complicated measures in carrying out the reaction.
Eutectics:
LiAlCl
4
—AlCl
3
m.p. 80° C.
NaAlCl
4
—AlCl
3
m.p. 113° C.
KAlCl
4
—AlCl
3
m.p. 133° C.
Mg(AlCl
4
)
2
—AlCl
3
m.p. 184° C.
It has therefore been found to be advantageous to use the (tetra)chloroaluminates whose vapor pressure becomes significant only far above the reaction temperatures to be employed according to the invention.
Examples are:
LiAlCl
4
m.p. 143° C.
NaAlCl
4
m.p. 153° C.
KAlCl
4
m.p. 256° C.
Mg(AlCl
4
)
2
m.p. 230° C.
Ca(AlCl
4
)
2
m.p. 218° C.
It has now surprisingly been found that pseudobinary and pseudoternary mixtures of these chloroaluminates are, owing to the favorable change in physical properties of the melt associated therewith, particularly advantageous for the preparation of and the direct hydrogenation using finely dispersed metals forming interstitial hydrides, for example the eutectics:
NaAlCl
4
—KAlCl
4
m.p. 125° C.
NaAlCl
4
—KAlCl
4
-AlCl
3
m.p. 89° C.
NaAlCl
4
—KAlCl
4
-MgCl
2
m.p. 125° C.
KAlCl
4
—Ca(AlCl
4
)
2
m.p. 148° C.
Preference is given to using the system NaAlCl
4
—KAlCl
4
in a
Liesenhoff Hans
Sundermeyer Wolfgang
Bos Steven
Degussa - AG
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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