The present invention relates to a novel process for producing anhydrous alkali sulfides which are useful as synthetic intermediates of sulfur-containing organosilicon compounds.

Since sodium polysulfide such as sodium tetrasulfide which is produced by combination of sodium hydrosulfide or sodium sulfide and sulfur (Inorganic and Theoretical Chemistry Vol. II, Longmans Green and Co., Ltd., (1961), p991) have high water absorption properties, it is not easy to obtain anhydrides thereof. A complex drying step is necessary in order to dehydrate these substances, which lowers the yield of the object substance.

Much energy is required in order to obtain anhydrous sulfides from hydrates of sulfides. In addition, since alkali polysulfides such as sodium disulfide and sodium tetrasulfide are viscous liquids under drying conditions, for example, at high temperatures of 120°to 130°C, it is difficult to treat them ( European Patent Publication No. 361,998 and Japanese Laid-open Patent Publication No. 228588/1995 ).

As another method of obtaining sodium sulfide, there is a process wherein sodium alkoxide is allowed to react with hydrogen sulfide ( United States Patent No. 5,466,848 and Japanese Laid-open Patent Publication No. 228588/1995 ). Good anhydrous sulfide is obtained by this process. On the other hand, since hydrogensulfide is toxic, it is necessary to treat it carefully, and an extra step is required in order to avoid poisoning.

Furthermore, Inorganic and Theoretical Chemistry, Vol. II, Longmans Green and Co., Ltd., (1961), p981discloses a process wherein metallic sodium is allowed to react with sulfur in liquid ammonia or xylene. However, the reaction using liquid ammonia is carried out at ultra-low temperatures, and treatment of ammonia is also troublesome. Since the reaction in an aromatic solvent such as xylene proceeds explosively at about 98°C, which is a melting point of the metallic sodium, the reaction is not practical.

In J. Inorg. Nucl. Chem., 1977, vol. 39, pages 1761-1766, Rauh et al. disclose a preparation method of lithium polysulfide by the direct reaction of sulfur with Li metal in an aprotic solvent.

US patent 5,039,506 relates to a method for preparing sodium sulfide from sodium and sulfur under protective gas, without requiring a reaction medium.

An object of the present invention is to provide a method of obtaining anhydrous alkali sulfides which are useful as synthetic intermediates of sulfur-containing organosilicon compounds in a simple and safe manner without treatment at ultra-low temperatures, high temperatures or high pressure in view of the above-mentioned problems of the prior arts.

As a result of studies to achieve the above-mentioned objects, the inventors found the following process for producing anhydrous alkali sulfides.

The process according to the present invention comprises reacting sulfur with metallic sodium or metallic potassium in the presence of an aprotic solvent.

In the process, sulfur is preferably anhydrous and preferably takes the form of powder or flake for high solubility in the solvent used. The smaller the particle diameter of sulfur having such a form, the higher does sulfur exhibit reaction efficency. It is preferable to select sulfur having appropriate particle diameter with respect to the solvent used in order to control the reaction. Since the reaction is exothermic, sulfur having large particle diameter is sometimes preferable.

The alkali metal is metallic sodium or metallic potassium and preferably metallic sodium.

In the process it is preferable to carry out the reaction under an inert gas atmosphere such as a nitrogen gas or argon gas atmosphere dried sufficiently.

The aprotic solvent can be a solvent which does not have an active hydrogen atom which reacts with the alkali metal, and it is preferable to use a solvent dried to the utmost in order to prevent hydrolysis of alkoxide due to water. Examples of the aprotic solvent which can suitably be used are aromatic hydrocarbons such as benzene, toluene and xylene, and ether solvents such as tetrahydropyran, dioxane and dibutyl ether. In particular, among these ethers, using a solvent which solvates the starting material or the object substance, the reaction is promoted. In this case, when crown ether or the like is further added to the solvent as a reaction accelerator, the reaction is much more promoted.

The preferred aprotic solvent in the process is a solvent which can solvate metallic sodium or metallic potassium, ion or dissolve alkali sulfide. The solvent which can solvate the alkali metal ion is a solvent which can solvate the alkali metal ion by formation of chelate with the ion. The reaction of sulfur with the alkali metal is promoted by using the solvent.

The solvents which can dissolve the alkali sulfide in the present specification can be not only the solvents which can always and perfectly dissolve the alkali sulfide which forms in the reaction system by the process for production, but also the solvents can dissolve the alkali sulfide deposited in the reaction system in the solvent successively and promptly under the reaction condition. So far as the solvents can dissolve the alkali sulfide of necessary and sufficient amount required for the progress of the reaction, the alkali sulfide can deposit in the reaction system.

Examples of such a solvent are tetrahydropyran, crown ether, and alkyl ethers of polyhydroxy alcohols such as dimethoxyethane (DME, ethylene glycol dimethyl ether), diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dibutyl ether and propylene glycol dimethyl ether. It is preferable to use a solvent dried to the utmost in order to prevent hydrolysis of alkoxide due to water. In general, a low boiling point solvent requires less energy to be recovered. However, it is particularly preferable to use dimethoxyethane, etc. as the solvent since a reaction temperature of the latter step by the halogenoalkoxysilane [I] is usually 60° to 100°C.

The anhydrous alkali sulfide which is the reaction product obtained by the reaction of sulfur and the alkali metal in the aprotic solvent can be taken out of the reaction liquid and used later, or the reaction liquid containing it can be used in the later step as it is.

The alkali sulfide is represented by the general formula (AIM)mSn (wherein AIM is an alkali metal, S is a sulfur atom, m is an integral number of 2 or 4, and n is an integral number of 1 to 9). However, some of the substance formed actually has composition distribution in a certain range. Examples of the alkali sulfide are sodium sulfide (Na₂S), sodium disulfide (Na₂S₂), sodium trisulfide (Na₂S₃), sodium tetrasulfide (Na₂S₄), potassium sulfide (K₂S), potassium trisulfide (K₂S₃), potassium pentasulfide (K₂S₅), potassium hexasulfide (K₂S₆), etc.

The present process is specifically described hereinafter by giving examples of the present invention. Unless the present invention deviates from the spirit thereof, the present invention is not limited to these examples.

A three-necked flask (200 ml) equipped with a condenser, a thermometer and a stirrer was first charged with 60 ml of dimethoxyethane (DME, water content : 1 ppm or lower) treated with a molecular sieve as an aprotic solvent while making dried nitrogen gas flow. To DME was added 6.4 g (0.2 mol) of powdered sulfur of 200 mesh, and then 4.6 g (0.2 mol) of metallic sodium was introduced into the flask at room temperature. Raising the temperature of the mixed liquid to about 65°C, a reaction of metallic sodium with sulfur began, and the mixed liquid became a uniform solution in about 15 minutes. Stirring was further continued at 70°C for 45 minutes to form anhydrous sodium sulfide.

The same three-necked flask (200 ml) as used in Example 1 was first charged with 60 ml of the same treated DME as used in Example 1 as an aprotic solvent while making dried nitrogen gas flow, and charged with 3.2 g (0.1 mol) of powdered sulfur of 200 mesh and then 4. g (0.2 mol) of metallic sodium at room temperature. Raising the temperature of the mixture to about 65°C, a reaction of metallic sodium with sulfur began. After 15 minutes, the mixed liquid became a uniform solution. The reaction temperature was further kept at 70°C, and the solution was heated while stirring it for 45 minutes. DME was evaporated under reduced pressure to give sodium sulfide.