Applicants are unaware of any prior art that describes the continuous, stoichiometric or virtually stoichiometric process described herein, or that describes application of the method to the production of silane from a silicon halide and a metal hydride.
Chemical reactions for the preparation of silane from metal hydrides are known in the art: ##STR1##
In addition to the reactions disclosed in the literature cited above, E.M. Marlett (U.S. Pat. No. 4,632,816, supra), discovered the reaction: EQU 5NaAlH.sub.4 +5SiF.sub.4 .fwdarw.Na.sub.5 Al.sub.3 F.sub.14 +2AlF.sub.3 +5SiH.sub.4 ( 7)
The Marlett patent, supra, discloses a process for the production of silane from a silicon tetrafluoride and a sodium aluminum tetrahydride or potassium aluminum tetrahydride, or mixture thereof. The reaction is carried out in a single reaction vessel, preferably it is suggested, by reacting a stoichiometric excess of the sodium aluminum tetrahydride, or potassium aluminum tetrahydride, with the total silicon tetrafluoride reactant. The stoichiometric excess of the metal hydride, it is suggested by the Patentee, is employed to eliminate boron impurities to produce a higher purity silane, and suppress the formation of undesirable products. The operation includes the use of a batch, or continuous back-mixed reactor, operated at conditions which causes the reaction to approach completion. This process, albeit a meritorious advancement in the state-of-the-art, leaves something to be desired. For example, the use in the reaction of excess sodium aluminum hydride, besides wasting valuable raw materials, results in the contamination of the sodium aluminum fluoride coproduct, a valuable material for use in the Hall aluminum process. The sodium aluminum floride during storage reacts with mositure to produce a hazardous condition. Worse perhaps, during the operation of the process the sodium aluminum hydride decomposes to form metallic aluminum which often creates maintainance problems by plating upon and fouling process equipment. Agitator blades and pumps as a result, e.g., have been damaged. Moreover, the silane product, due to its high solubility in the solvents, has been lost to waste; and flashing off the solvent provides only a partial solution to this loss, as well as resulting in the production of additional metallic aluminum by decomposition of the sodium aluminum hydride.
Application Ser. No. 075,367, supra, discloses a process for the continuous production of silane, and a halogenated coproduct, by the reaction of a silicon halide, notably silicon tetrafluoride, with a metal hydride, notably an alkali metal hydride or alkali metal aluminum hydride. The reaction, which is exothermic, is conducted by transfer of the reactants, in a slurry, between two separate reaction zones, a first generally relatively large volume reaction zone wherein the metal hydride is reacted with less than a stoichiometric quantity of the silicon halide, and a second generally relatively small volume reaction zone wherein the unreacted metal hydride, or metal hydride remaining in the slurry, is reacted with a quantity of silicon halide in excess of that required to react with the unreacted, remaining metal hydride. In the overall operation, a substantially stoichiometric reaction and high stability are achieved by feeding the same molar amounts of the metal hydride and silicon halide to the two reaction zones while regulating and controlling the flow of reactants between the two reaction zones. In accordance with this control scheme, the difference between the temperature of the reaction medium of the secondary reaction zone and the inlet temperature to the secondary reaction zone is sensed, and the differential temperature is used to regulate and control the flow of silicon halide to the secondary reaction zone.
The stoichiometric, staged reaction sequence has eliminated certain drawbacks of the Marlett process. For example, the chemical raw materials loss has been curtailed, and safety hazards reduced. The aluminum plating problem has been largely eliminated, and there is less contamination of the sodium aluminum fluoride coproduct with sodium aluminum hydride. Albeit however the process of application Ser. No. 075,367 represents a noteworthy achievement in having advanced the state-of-the-art relating to the production of silane, and a higher purity fluoride salt coproduct, further improvements are desirable. For example, it remains an objective to maintain the very low level of sodium aluminum hydride in the sodium aluminum fluoride coproduct, yet reduce the level of siliceous impurities in the sodium aluminum fluoride. Thus, the sodium aluminum fluoride product is yet often contaminated with silicates, at least one of which has been identified as Na.sub.2 SiF.sub.6, generally analytically reported with other siliceous impurities as SiO.sub.2. The latter impurity forms as the result of a secondary reaction between the silicon tetrafluoride and the sodium aluminum fluoride coproduct, this differing of course from the primary reaction between the silicon tetrafluoride and the sodium aluminum hydride reactants which forms silane, and the coproduct sodium aluminum fluoride.
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