Patent Application
20120321540
This disclosure relates to a method for producing oligosilanes.
Oligosilanes are usually produced by hydrogenation of halogenated silicon compounds with metal or semimetal hydrides. Methods reliant on high reaction temperatures, for example, hydrogenation of halogenated silicon compounds in the absence of solvent are not very suitable for preparing oligosilanes. At high reaction temperatures, decomposition of a silicon compound or of an oligosilane can occur with dissociation of a monosilane, for example. Such dissociation can be effected, for example, as a result of a reductive cleavage of a Si—Si bond. Methods that are sufficiently fast at low reaction temperatures need readily soluble complex metal hydrides. These complex metal hydrides, for example, metal borohydrides or metal aluminum hydrides, are more soluble than alkali or alkaline earth metal hydrides, but lead to a significant increase in manufacturing costs.
Methods for producing oligosilanes are therefore desired to be simpler and more economical to carry out and provide oligosilanes in high purity. It could therefore be helpful to provide an improved and more economical method for producing oligosilanes.
We provide a method for producing oligosilanes by reacting halogenated oligosilanes with a metal hydride, wherein the reaction takes place in the presence of a catalyst and of an alkali metal halide, the catalyst comprises a halide of a multivalent metal, and the reaction takes place in an ethereal solvent.
We provide a method for producing oligosilanes. Oligosilanes are produced according to this method by reacting halogenated oligosilanes with a metal hydride, wherein
“Halogenated oligosilanes” are predominantly or completely substituted with halogen atoms. “Oligosilanes” are predominantly or completely substituted with hydrogen atoms. The halogenated oligosilanes are reduced by our method to oligosilanes. The oligosilanes and halogenated oligosilanes may each be mixtures of compounds or single compounds. Oligosilanes and halogenated oligosilanes are compounds which either have a single central silicon atom or, if they have two or more silicon atoms, they are interconnected by Si—Si bonds. More particularly, oligosilanes and halogenated oligosilanes have from 1 to 8 silicon atoms.
A multivalent metal has an oxidation number>1. Alkali metals are not multivalent metals, their oxidation number in halides is=1. Alkali metal halides herein are not counted as catalysts. A reaction mixture may be a solution and more particularly a suspension and may be through-mixed by shaking or stirring.
The alkali metal halide serves inter alia to improve the solubility of the metal hydride and of other reactive species so that their reactivity is enhanced, or to convert the catalyst into a more reactive form. It is thus the case that in the presence of the alkali metal halide, conversion of halogenated oligosilanes into oligosilanes is accelerated compared with a conventional method in the absence of such an alkali metal halide. This makes it possible to use smaller amounts of catalyst which can act as a Lewis acid. Yet the reaction rate is not adversely affected by the low concentration of catalyst and may even be enhanced. Thus, in our method, the amount of catalyst added can be reduced by the less costly alkali metal halide, to thereby lower manufacturing costs. Alkali metal halides are ecologically unconcerning, and so the method is not just more economical than conventional methods, but also has a lesser environmental impact.
Combining catalyst and alkali metal halide makes it possible to perform conversion of halogenated oligosilanes to oligosilanes at low reaction temperatures. As a result, undesired decompositions such as dissociation of monosilanes for example, can be reduced or almost completely prevented. Almost completely is to be understood as meaning to an extent of at least 95% and more particularly to an extent of at least 99%. Since less decomposition occurs, the overall yield of the method increases, making it more economical. Advantageously, the desired oligosilanes are also obtained in high purity, since they are less contaminated with decomposition products than is the case with conventional methods.
Oligosilanes are generally volatile and advantageously removable from the reaction mixture via the gas phase and are thus easy to isolate.
The catalyst may be used in superstoichiometric quantity relative to the metal hydride. That is, more than one equivalent of catalyst can be used per equivalent of metal hydride.
The catalyst may be used in a stoichiometric or a substoichiometric amount relative to the metal hydride. The catalyst may preferably be used in a substoichiometric amount relative to the metal hydride. “Substoichiometric” amount is to be understood as meaning an amount of less than one equivalent. That is, less than one equivalent of catalyst per equivalent of metal hydride can be used in the method.
The metal hydride may be used in a molar ratio to catalyst ranging from 1:1 to 200:1. The metal hydride can be used in a molar ratio to catalyst ranging from 4:1 to 150:1, preferably from 20:1 to 100:1. In particular, the combination of the catalyst with the alkali metal halide makes it possible to use particularly small amounts of the catalyst in the method.
The alkali metal halide may be used in a substoichiometric amount relative to the metal hydride.
The catalyst may be used in a ratio to alkali metal halide of 1:4 to 10:1 and preferably 1:3 to 2:1. Examples of the method in which only small amounts of alkali metal halide are needed are particularly advantageous. Since both the catalyst and alkali metal halide can be used in substoichiometric amounts, the apparatus requirements of the method are also reduced. For example, the reaction mixture is easier to through-mix in those cases where the catalyst and/or the alkali metal halide are not present in solution or only present to a small degree. This in turn facilitates the method.
The metal hydride may be used in a molar ratio to halide contained in the halogenated oligosilane of 1:1.1 to 5:1 and particularly 1:1 to 1.5:1. Advantageously, therefore, the method requires only stoichiometric or slightly superstoichiometric amounts of metal hydride per halide in the silane, even in the presence of small amounts of catalyst. The molar amount of halide in the halogenated oligosilane is greater than the molar amount of the halogenated oligosilane. For example, the molar amount of chloride in the halogenated oligosilane Si2Cl6 is equal to six times the molar amount of Si2Cl6 used.
A metal hydride may be used which comprises or consists of an alkali metal hydride, an alkaline earth metal hydride or a combination thereof. The alkali metal hydride used can be lithium hydride (LiH), sodium hydride (NaH), potassium hydride (KH) and a combination thereof. The alkaline earth metal hydride used can be magnesium hydride (MgH2), calcium hydride (CaH2) and a combination thereof.
In general, alkali metal hydrides or alkaline earth metal hydrides are less costly than the complex hydrides, for example, metal borohydrides or metal aluminum hydrides obtained therefrom. Therefore, our method is more economical than conventional methods involving complex metal hydrides since the alkali metal and/or alkaline earth metal hydrides are activated by adding substoichiometric amounts of catalyst compounds and alkali metal halides. Catalyst compounds are more particularly the catalysts recited hereinbelow. Examples of complex hydrides are sodium borohydride, lithium aluminum hydride and Red-Al®.
Advantageously, alkali metal hydrides and alkaline earth metal hydrides are not just less costly than complex hydrides, they are also easier to handle. Some alkali metal hydrides are commercially available at low cost in the form of a dispersion in organic solvents and can also be used in our method in the form of that dispersion. The metal hydride in such dispersions is well protected from moisture and atmospheric oxygen. This provides more particularly a technical advantage over conventional methods involving complex metal hydrides, which are very sensitive to moisture and/or atmospheric oxygen. This also makes the method safer since the risk of upsets due to spontaneous, uncontrolled side reactions is reduced. A further advantage of alkali metal hydrides and alkaline earth metal hydrides is that, unlike borohydrides or metal borohydrides, they cannot release any volatile toxic compounds such as diborane.
A metal hydride may be used which comprises or consists of an alkali metal hydride. More particularly, the alkali metal hydride may comprise or consist of lithium hydride. Advantageously, lithium hydride is very inexpensive and has a low molecular weight. Furthermore, the mass fraction of hydride is very much larger in lithium hydride than in other metal hydrides. Therefore, the mass of metal hydride used can be reduced compared with other metal hydrides, reducing manufacturing costs and waste. Lithium hydride is advantageously also easy to handle in the form of the pure solid material.
The reaction may be carried out in an ethereal solvent comprising or consisting of a first solvent having an ether group and optionally a second solvent. Nonlimiting examples of first solvents, which contain at least one ether group, are diethyl ether, tetrahydrofuran, dipropyl ether, butyl methyl ether, dibutyl ether, diphenyl ether, dioxane, dimethoxyethane or diethylene glycol dimethyl ether. The ethereal solvent may also comprise or consist of a combination of first solvents. Therefore, the ethereal solvent may contain at least one first solvent having an ether group, also called ether function, or consist thereof. A first solvent having an ether function is important for an efficient reaction because it improves solubility and/or is important for stabilizing some species in the reaction mixture.
Nonlimiting examples of the second solvent which does not contain an ether group, optionally present in the ethereal solvent are aromatic compounds such as toluene, xylene, ethylbenzene or alkanes such as octane, decane or paraffin oil and also mixtures thereof. It is also possible for the reaction mixture to contain a mineral oil as a second solvent.
In the mentioned first and second solvents, the alkyl substituents also represent branched alkyl substituents. Propyl thus represents both n-propyl and isopropyl, which means that dipropyl ether, for example, is representative of di-n-propyl ether, n-propyl isopropyl ether and diisopropyl ether. Similarly, butyl represents each of n-butyl, sec-butyl, tert-butyl and isobutyl. The solvents are further also representative of the entire family of solvents, i.e., xylene is representative of ortho-, meta- and para-xylene, octane is representative of n-octane or a branched octane, decane is representative of n-decane or a branched decane, or the like.
Solvents used more particularly in the method have a higher boiling point and hence a lower vapor pressure than the desired product, the oligosilane. As a result, the solvent does not evaporate as readily as the desired reaction product, and so the oligosilane is preferentially removable from the reaction mixture via the gas phase. The oligosilane is thus easy to isolate. An example of a possible reaction product is the oligosilane Si3H8, which has a boiling point of about 60° C. under standard conditions. To produce Si3H8 from Si3Cl8, therefore, it is preferable to select a solvent having a boiling point of >60° C.
The catalyst may be a halide of a multivalent metal selected from the group consisting of: aluminum fluoride, aluminum chloride, aluminum bromide, aluminum iodide, gallium fluoride, gallium chloride, gallium bromide, gallium iodide and a combination thereof. A catalyst is used more particularly, which comprises or consists of aluminum chloride, aluminum bromide and a combination thereof. These inorganic catalysts are generally more economical than boron, aluminum or gallium compounds comprising organic substituents, wherein these organic substituents may be alkyl, aryl or alkoxy substituents for example. The inorganic catalysts used in the method are not just less costly than these organically substituted compounds, but they are also easier to handle, since they are not pyrophoric and not so sensitive to moisture and/or atmospheric oxygen. As a result, the above-described advantages are likewise achieved for our method.
Aluminum chloride may be used as catalyst. We realized that aluminum chloride in conjunction with an alkali metal halide can be used without further catalysts, which are generally costlier than aluminum chloride. This makes our method more economical than conventional methods. For example, using pure aluminum chloride is less costly than using aluminum bromide since the latter is significantly more expensive. It is therefore thus possible for our method to be more particularly less costly to carry out than other, conventional methods.
An alkali metal halide may be used, which is selected from the group consisting of: lithium chloride, lithium bromide, lithium iodide, sodium chloride, sodium bromide, sodium iodide and a combination thereof. It is more particularly possible to use an alkali metal halide selected from lithium chloride, lithium bromide and a combination thereof. It is more particularly possible to use lithium bromide as alkali metal halide since it is more soluble in ethereal solvents and, hence, is more reactive than lithium chloride. It is particularly advantageous to have a combination of substoichiometric amounts of lithium bromide with substoichiometric amounts of aluminum chloride as catalyst for the method.
Halogenated oligosilanes may have a composition represented by the formula SinHpXm−p. Here n=1 to 8, m=2n to 2n+2, 0≦p<0.5*m and X=F, Cl, Br, I or a combination thereof. More particularly, it is possible for X=F, Cl or a combination thereof. It is possible for n=1 to 5 and more particularly also n=2 to 5. It may be preferable for 0≦p<0.1*m and more particularly preferable for 0≦p<0.01*m to apply. The method may utilize not only linear, branched but also cyclic halogenated oligosilanes in order that the corresponding linear, branched as well as cyclic oligosilanes may be produced.
The halogenated oligosilanes are oligochlorosilanes which have a composition represented by the formula SinClm. In this formula, m=2n to 2n+2. More particularly, m=2n+2, so that the oligochlorosilanes are represented by the formula SinCl2n+2. Here it is possible for n=1 to 8, preferably n=1 to 5 and more preferably n=1 to 3.
Oligosilanes are produced from the halogenated oligosilanes having a composition represented by the formula SinHqXm−q, where 0.95*m≦q≦m and more particularly 0.99*m≦q≦m. It is possible for n and m to be selected as described above.
The reaction may take place at a pressure of 10 hPa to 1500 hPa. The reaction can take place more particularly at a pressure of 10 hPa to 500 hPa. More particularly, the reaction can take place at an underpressure, i.e., at below 1000 hPa, as a result of which the reaction product, the oligosilane, is easy to remove from the reaction mixture and, hence, easy to isolate. An underpressure reaction further makes it possible to remove the reaction product from the reaction mixture at low temperatures via the gas phase.
The reaction takes place at a temperature between −20° C. and the boiling point of the solvent. The reaction can also take place at −10° C. to 70° C. and more particularly 0° C. to 40° C.
The method may comprise the steps of:
The solvent may have an ether group, i.e., be an ethereal solvent as described above, or a further solvent having an ether group can be added in some other step, so that altogether an ethereal solvent is used in the method. The order of the reaction steps mentioned may be any desired, but this order is adopted in particular. Individual reaction steps may take place at the same time or be carried out together. The individual constituents of the reaction mixture may be chosen in accordance with the examples described.
The metal hydride can be added in step (a) either in pure form, as a suspension in an ethereal solvent or as a dispersion in an organic solvent. In step (b), the catalyst and the alkali metal halide can be added as solids, in dissolved form and/or as a suspension. The two compounds can be used separately from each other or as mixture. When the two compounds are added separately from each other, the order in which they are added is freely choosable.
The reaction may take place under mixing of liquid and solid phase, for example, by shaking or stirring, more particularly by stirring. Mixing can take place in one or more steps.
The halogenated oligosilane may be added in step (c) by metered addition. That is, the oligosilane is more particularly added in controlled fashion.
The oligosilane formed may be removed from the reaction mixture in gaseous form in a further step (d). Isolating and enriching can take place separately. Step (d) can take place simultaneously or partly simultaneously with one or more other steps, more particularly simultaneously or partly simultaneously with step (c).
In the following, a further example of the method for the production of oligosilanes by reacting halogenated oligosilanes with a metal hydride is described, which combines multiple examples of the method. This version may be more particularly complemented in any desired manner with the further recited examples.
To produce oligosilanes SinH2+2 (n=1-3) from respectively tetrachlorosilane SiCl4, hexachlorodisilane Si2Cl6 or octachlorotrisilane Si3Cl8, alkali metal hydride is initially charged in a solvent which contains at least one ether group or mixtures of solvents which each contain at least one ether group or mixtures of at least one solvent which contains at least one ether group with further solvents which contain no ether groups, and AlCl3 and lithium bromide LiBr are added. The oligochlorosilane SinCl2+2 (n=1-3) is metered into this mixture and the respective reaction product SiH4, Si2H6 or Si3H8 is removed from the reaction space in gaseous form.
Nonlimiting examples of solvents containing at least one ether group are diethyl ether, tetrahydrofuran, dipropyl ether, butyl methyl ether, dibutyl ether, diphenyl ether, dioxane, dimethoxyethane or diethylene glycol dimethyl ether. Nonlimiting examples of solvents containing no ether group are aromatic compounds such as toluene, xylene, ethylbenzene or alkanes such as octane, decane or paraffin oil and also mixtures thereof. Preference is given to using solvents having a higher boiling point than the particular product. Accordingly, Si3H8 is produced using solvents or solvent mixtures having an atmospheric pressure boiling point >60° C.
Examples of alkali metal hydride are LiH, sodium hydride (NaH), potassium hydride (KH) or mixtures thereof. Preference is given to using LiH.
AlCl3 and LiBr can be added as solids or in dissolved form. The two compounds can be used separately from each other or as a mixture. When the two compounds are added separately from each other, the order of addition is freely choosable.
The reaction is preferably carried out under mixing of liquid and solid phase, for example, by stirring.
The silanes are produced at temperatures between −20° C. and the boiling point of the solvent or solvent mixture used. The reaction temperature is preferably −10° C. to 70° C. and more preferably 0° C. to 40° C.
The method is carried out at a pressure of 10 hPa to 150 kPa. The production of Si3H8 at reduced pressure of 10 hPa to 50 kPa is preferred.
The molar ratio of AlCl3 used to LiBr used is 1:4 to 10:1 and preferably 1:3 to 2:1.
The methods described here are not limited by the description with reference to the operative examples. On the contrary, this disclosure comprises every novel feature and also every combination of features, which more particularly includes every combination of features in the appended claims, even if that feature or that combination itself is not explicitly indicated in the claims or operative examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 056731.3 | Dec 2009 | DE | national |
This is a §371 of International Application No. PCT/EP2010/068995, with an international filing date of Dec. 6, 2010, which is based on German Patent Application No. 10 2009 056 731.3, filed Dec. 4, 2009, the subject matter of which is incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP10/68995 | 12/6/2010 | WO | 00 | 9/7/2012 |
