2,992,073 United States Patent Office Patented July 11, 1961 1. 2 chloride corresponding to the alkali metal fluoride em 2,992,073 ployed. In Reactions 4 and 5, there are formed in addi SYNTHESS OF SULFURTETRAFLUORDE Charles W. Tullock, Wilmington, Del, assignor to E. I. tion sulfur monochloride and sulfur, respectively. Reac du Pont de Nemours and Company, Wilmington, Dei, tion 6 uses the same reactants as Reaction 1 but with a corporation of Delaware an excess of sulfur, and it has been found that sulfur No Drawing. Filed Mar. 12, 1959, Ser. No. 798,828 monochloride is formed in such a system. 18 Claims. (CI. 23-205) Instead of sulfur or a sulfur chloride, it is also possi ble to use chlorine and one of a class of Sulfur-supplying This invention relates to a new process for the syn reactants, the binary sulfides, which, in contact with thesis of sulfur tetrafluoride. 0 chlorine under the operating conditions, form sulfur This application is a continuation-in-part of my ap and/or a sulfur chloride. Such sulfur-supplying react plication Ser. No. 664,154, filed June 7, 1957, in turn a ants include the sulfides of metals of groups I and II continuation-in-part of my application Ser. No. 603,880, of the periodic table and of those metals in group VIII filed August 14, 1956, and both now abandoned. 5 which have atomic numbers from 26 to 28. Suitable Sulfur tetrafluoride (sulfur (IV) fluoride, SF) has sulfides are, for example, the sulfides of sodium, potas not been extensively studied, primarily because the meth sium, calcium, barium, iron, cobalt, nickel, copper, zinc, ods of synthesizing it known heretofore (see the article cadmium and mercury. The alkali metal sulfides are by Brown and Robinson in J. Chem. Soc. 1955, 3147) preferred for this purpose. With an alkali metal sul are all based ultimately on the use of elemental fluorine, 20 fide, the overall reaction may be represented by the and are therefore expensive and poorly adapted to large equation: scale operations. It is known (U.S. Patent 2,709, 186) that Sulfur tetrafluoride can be converted to the tech nically highly valuable tetrafluoroethylene by reaction where M and M represent alkali metals which need with carbon at high temperatudes, e.g., by passage 25 not be the same. This embodiment of the invention is through a carbon arc. This and other uses for sulfur illustrated in Example XXII, infra. tetrafluoride have not been exploited because of difficul It is by no means essential to use the reactants in ties in preparing this compound such as those mentioned. precisely the stoichiometrical proportions shown in the An object of this invention is, therefore, provision of above equations. In fact, it is in general desirable to a novel method for synthesizing sulfur tetrafluoride. 30 operate, in all systems, with a slight to moderate excess Another object is provision of a method for syn of alkali metal fluoride over the amounts shown, e.g., a thesizing sulfur tetrafluoride which does not involve 5-50% excess. However, in the interest of efficient the use of elemental fluorine. utilization of the reactants, it is recommended to have The above-mentioned and yet other objects are 35 the chlorine and the sulfur present in the system (sepa achieved in accordance with this invention by a process rately or combined) in such amounts that the atom ratio in which an alkali metal fluoride of atomic number 11 Cl:S is at least 1:1. Since sulfur monochloride, where to 55, inclusive, is reacted with chlorine and sulfur from the Cl:S ratio is 1:1 (when no additional chlorine is suitable chlorine- and sulfur-supplying reactants under used) is much less effective than the other systems, it Substantially anhydrous conditions and at a temperature 40 is preferred to use the chlorine and the sulfur (sepa of at least 20° C., whereby sulfur tetrafluoride is pro rately or combined) in an atom ratio Cl:S of at least duced. 1.5:1, and still more preferably at least 2:1. In this reaction, the chlorine and the sulfur can be Any alkali metal fluoride in anhydrous form can be employed either in elemental or in combined form. 45 used, in which fluoride the alkali metal has an atomic Thus, the reaction mixture can comprise, in addition to number from 11 to 55, inclusive. Cesium and rubidium the alkali metal fluoride, sulfur and chlorine as the sepa fluorides are the most reactive, but they are not readily rate elements; or sulfur dichloride, SC, with or with accessible. Sodium and potassium fluoride are the most out additional chlorine; or sulfur monochloride, SaCl2, useful and preferred reactants. with or without additional chlorine. 50 The process can be carried out with or without the While the mechanism of the reaction is not known use of a liquid reaction medium, with substantially equiv with certainty, it appears that the stoichiometry of the alent results as regards conversions to sulfur tetrafluoride. process, using the above-mentioned sets of reactants, can However, the use of certain specific reaction media, char be represented by the following equations, wherein MF acterized by a high dielectric constant, has a marked denotes an alkali metal fluoride, effect on the reaction temperature. As more fully dis cussed below, these special media, particularly when used with Sulfur dichloride as the sulfur-chlorine reactant, per mit conducting the reaction at temperatures as low as the ordinary ambient temperature of about 20° C. 60 When no diluent is used, or when an organic liquid of low dielectric constant (below about 20) is used as the reaction medium, the appropriate reaction temperature In Reactions 1, 2 and 3 above, the sole primary reac depends chiefly on the reactivity of the alkali metal fluo tion products are sulfur tetrafluoride and the alkali metal 65 ride. A highly reactive fluoride such as cesium fluoride - 2,992,073. is - - 3. 4. reacts with sulfur and chlorine at temperatures as low as sulfate, cyclic tetramethylene sulfone, ethylene carbonate, about 80° C. The less reactive sodium fluoride normally etc. The preferred reaction media, because of their inert requires heating to at least 125 C., preferably 150 C. ness towards the chlorine-containing reactant under the or above to secure a practical reaction rate. The upper operating conditions, are the nitriles, N,N-disubstituted limit of temperature, with any set of reactants, is dictated 5 carbonamides, nitro compounds, sulfones and alkyl Sul chiefly by the fact that, at high temperatures, sulfur tetra fates which, except for the functional groups, are hydro fluoride disproportionates to sulfur hexafluoride and sul carbon. Otherwise stated, the preferred reaction media fur, and also by the increased corrosive effect of the re are those compounds, having dielectric constant of at least actants on the usual materials of construction. For these 20 at 20° C., which contain only carbon, hydrogen and reasons, temperatures above 400° C. are not recom O one of the groups mended, although appreciable amounts of Sulfur tetra fluoride are formed at temperatures up to 600 C. in -CN, --NK -NO-SO-o-, and -O-SO-O- general, the best conversions are obtained at temperatures below 300 C., and such temperatures are therefore pre ferred. 5 When it is not desired to use one of the above-discussed As already mentioned, reaction media of a specific high dielectric constant reaction media, the reaction is type permit carrying out the process at low or very mod preferably conducted without added diluent. However, erate temperatures, e.g., in the range of 20-150° C., and the presence of an organic liquid of low dielectric con preferably 40-90 C, using sulfur dichloride, alone or stant, while it offers no special advantages, does no harm, with excess chlorine, as the sulfur-chlorine reactant. 20 provided the medium is substantially inert towards the These particular media are organic compounds, liquid reactants and towards sulfur tetrafluoride at the relatively under the operating conditions, which have a high di high temperatures which are generally necessary. Among lectric constant, at least 20 when measured at or above suitable diluents of this kind may be mentioned in par 20 C. The function of these high dielectric constant ticular the perfluorinated hydrocarbons such as perfluoro media is not clearly understood. It is possible that they 25 n-heptane, perfluoro-n-decane, perfluoro(methylcyclo solvate the chlorine atoms, making them more reactive, hexane), and the like. As already mentioned, it is in or that they ionize the sulfur dichloride. The suitability general necessary, in the absence of a high dielectric con of a liquid medium of this type is not connected with its stant reaction medium, to operate at higher temperatures, ability to dissolve the alkali metal fluoride since many of for example in the range of 100-300° C., and preferably them have little or no solvent action on these salts. How 30 150-250° C. ever, the reaction medium should be one in which the sui When a high dielectric constant reaction medium is fur chloride is at least partly soluble, for example to the used, the process can be carried out at or near atmos extent of 20% by weight. Preferably, the reaction me pheric pressure, the sulfur tetrafluoride (a gas boiling at dium is miscible with the sulfur chloride. In using these about -40° C.) being permitted to escape from the re special reaction media at the low reaction temperatures 3 5 action mixture and being led to cold receivers where it is which they make possible, sulfur dichloride, alone or condensed.