3,112,180 United States Patent Office Patiented Nov. 26, 1963 2 borohydride and diborane gas is evolved. The preferred 3,12,130 method of conducting the reaction is to dissolve the weak METHOD FOR THE PREPARATION OF D30RANE inorganic acid, salt or ester in the sulfuric or chloro Rosert D. Schaatz, East Whittier, Calif., assig or to sulfonic acid, with heating if necessary to effect solution. Aerojet-General Corporation, Azusa, Calif., a corpora The reaction is usually conducted in a reactor equipped tion of Ohio with a gas outlet leading to a low temperature trap. No Drawing. Filled Mar. 10, 1955, Ser. No. 493,570 The acid solution is placed in the reactor and finely 17 Caians. (C. 23-224) divided borohydride added with agitation. During the This invention relates to a new and improved method course of the reaction, a gentle evolution of hydrogen of preparing diborane. IO) and diborane gases takes place. These gases are vented Diborane is an excellent high energy fuel. Unfor through the gas outlet and collected in the low tempera tunately, however, its use has been severely limited due ture trap. If desired, the reaction can also be conducted to a lack of a convenient and inexpensive method for in vacuum or a nitrogen atmosphere as well as in air. preparing it in high yield. To more clearly illustrate this invention, the following Heretofore, diborane has been prepared by reacting examples are presented. It is to be understood, however, alkali metal borohydrides with hydrogen chloride gas that these examples are presented merely as a means of at room temperature or with liquid hydrogen chloride illustration and are not intended to limit the scope of at a temperature of -78° C. These methods are very the invention in any way. slow, requiring reaction times of from 16 to 24 hours. EXAMPLE I in addition, the separation of diborane from the result 20 ing 70% diborane-30% hydrogen chloride azeotropic Fluorinated Phosphoric Acid Moderation mixture is very difficult. Anhydrous monofluorophosphoric acid in an amount It has long been desired as a matter of cost, con of 10% by volume was dissolved in 100 cc. concentrated venience and commercial feasibility, to produce diborane sulfuric acid. The solution was placed in a 3-necked by using sulfuric acid as the protolyzing acid, prefer 25 flask equipped with a wire stirrer, a gas outlet leading ably at room temperature. Unfortunately, when alkali to a low temperature trap, and a dropping funnel. The metal borohydrides were reacted with concentrated sul reactor was swept out with nitrogen and a nitrogen gas furic acid instead of diborane, boron oxides were pro flow was maintained through the dropping funnel. 5 duced with explosive violence. In an attempt to avoid gm. of powdered sodium borohydride was dropped into oxidation, dilute Sulfuric acid was substituted in the re 30 action. Although the borohydride-dilute Sulfuric acid the acid with rapid stirring. The reaction proceeded reaction was not explosive, hydrolysis occurred producing smoothly and quietly with no sparking or flaming. Visi boric acid instead of diborane. ble evolution of gas ceased in less than 10 minutes. Di We have now found that diborane is rapidly and safely borane was recovered in a yield of more than 80%. produced in good yield by reacting a metal borohydride 35 EXAMPLE II with concentrated sulfuric or chlorosulfonic acid in the presence of a weak, inorganic nonoxidizing acid or with Dibasic Sodium Phosphate Moderation an alkali metal salt or alkyl ester of such an acid, in 1 cc. of a solution of 10% by weight of dibasic sodium accordance with the general reaction scheme set forth phosphate, dissolved in concentrated Sulfuric acid of below: 95% purity, was placed in a test tube and an 8-mesh crystal of sodium borohydride was added to the acid wherein M is a metal radical. Due to the non-volatile salt mixture. Diborane was gently evolved. characteristics of sulfuric acid, an azeotropic mixture of EXAMPLE II gases is avoided. Although rapid, the reaction proceeds smoothly and gently without sparking, flaming, or ex 45 Boric Acid Moderation plosions. Boric acid in an amount of 10% by weight was dis Suitable modifiers are the phosphoric acids such as solved in concentrated sulfuric acid. 50 cc. of the acid orthophosphoric, metaphosphoric and pyrophosphoric mixture was placed in a 3-necked flask fitted with a wire acids; the fluorinated phosphoric acids such as mono stirrer, a gas outlet leading to a low temperature trap, fluoro and difluorophosphoric acid; boric acid; the alkali 50 and a dropping funnel. The reactor was swept out with metal salts of the above acids such as sodium dihydrogen nitrogen and a nitrogen gas flow was maintained through phosphate, potassium phosphate, sodium difluorophos the dropping funnel. 1 gm. of finely divided sodium phate and lithium borate; and the alkyl esters of the borohydride was dropped into the acid mixture with stir above acids such as triquatradecyl borate, tridodecyl ring. A gentle evolution of gas occurred and a 73% yield borate, tridodecyl phosphate, pentyl difluorophosphate 55 of diborane was recovered in the low temperature trap. and diheptyl phosphate. As a matter of convenience, alkali and alkaline earth EXAMPLE V metal borohydrides such as potassium and lithium boro Boric Acid Moderation hydride are usually employed as starting materials. Com merical grade sulfuric acid, ordinarily containing from 60 Using the apparatus described in Example III, 5 gm. about 95% to about 98% by weight pure hydrogen sul of sodium borohydride was reacted with 70 cc. of 90% fate and about 5% to about 2% by weight water is or sulfuric acid and 10% boric acid. Formation of a heavy dinarily used as the protolyzing acid. Chlorosulfonic white precipitate occurred and diborane was recovered acid decomposes in water and therefore it is preferred in a 53% yield. that the system be anhydrous when this acid is em 65 EXAMPLE V ployed. Mixtures of chlorosulfonic and sufuric acid Boric Acid Moderation may also be used if desired. In accordance with the present invention, sulfuric or A 1 cc. sample of concentrated sulfuric acid containing chlorosulfonic acid containing a weak, inorganic non 10% by weight boric acid was placed in a test tube, oxidizing acid, or an alkali metal salt or alkyl ester of 70 and a 8-mesh crystal of sodium borohydride was dropped such an acid, usually in amounts of from about 2% to into the solution. A gentle evolution of gas occurred about 10% by weight of the acid, is reacted with a metal and diborane was identified by its infrared spectrum. 3,112,180 3 4. EXAMPLE VI 7. The method of claim 1 wherein the said modifier is tridodecyl borate. Tridodecyl Borate Moderation 8. The method of claim 1 wherein the said modifier To a 10 cc. sample of concentrated sulfuric acid of is tri-n-butyl borate. 97% purity was added 2 drops of tridodecyl borate. An 9. The method of claim 1 wherein said protolyzing 8-mesh sodium borohydride crystal was dropped into acid and said modifier are first mixed together and said the solution. Gentle evolution of gas occurred and di metal borohydride then added to the mixture. borane was identified by its infrared Spectrum. 10. A method of preparing diborane which comprises EXAMPLE VI reacting sodium borohydride with concentrated sulfuric 0 acid in the presence of from about 2% to about 10% Tri-n-Butyl Borate Moderation monofluorophosphoric acid by weight of the sulfuric acid. A 1 cc. sample of concentrated sulfuric acid contain 11. A method of preparing diborane which comprises ing 5% by weight tri-n-butyl borate was placed in a test reacting sodium borohydride with concentrated sulfuric tube and an 8-mesh crystal of sodium borohydride was acid in the presence of from about 2% to about 10% dropped into the solution. A gentle evolution of gas orthophosphoric acid by weight of the sulfuric acid. occurred and diborane was identified by its infrared spec 12. A method of preparing diborane which comprises trum. reacting sodium borohydride with concentrated sulfuric Optimum results are obtained by first dissolving the acid in the presence of from about 2% to about 10% modifier in the protolyzing acid and subsequently adding dibasic sodium phosphate by weight of the sulfuric acid. the metal borohydride to the solution. This procedure 20 13. A method of preparing diborane which comprises provides a homogeneous reaction mixture in which max reacting sodium borohydride with concentrated sulfuric imum protection of the metal borohydride particles from acid in the presence of from about 2% to about 10% the oxidizing influence of the protolyzing acid is obtained boric acid by weight of the sulfuric acid. as well as maximum surface area for reaction. It is 14. A method of preparing diborane which comprises often desirable to heat the modifier-protolyzing acid mix reacting sodium borohydride with concentrated sulfuric ture to effect solution, however the temperature at which acid in the presence of from about 2% to about 10% the diborane producing reaction proceeds is not limited tridodecyl borate by weight of the sulfuric acid. by this expedient. The reactants and products of the 15. A method of preparing diborane which comprises reaction are thermally stable and the temperature at reacting sodium borohydride with concentrated sulfuric which the reaction is conducted is not critical, although 30 acid in the presence of from about 2% to about 10% as a matter of convenience the reaction is conducted tri-n-butyl borate by weight of the sulfuric acid. at about room temperature. 16. A method of preparing diborane which comprises I have invented a means of moderating the reaction reacting sodium borohydride with concentrated sulfuric of borohydrides with sulfuric or chlorosulfonic acid so acid in the presence of about 10% orthophosphoric acid that diborane is produced safely, rapidly, and convenient by weight of the sulfuric acid. ly. Due to the inexpensiveness and availability of sul 17. A method of preparing diborane which comprises furic acid, ease of running the reaction at ordinary ten reacting sodium borohydride with concentrated sulfuric peratures, and the convenience of recovering diborane acid admixed with a reaction moderating amount of from the reaction mixture, the above described method orthophosphoric acid. of preparing diborane will find valuable use in the produc 40 tion of this high energy fuel. As well as being useful References Cited in the file of this patent as a high energy fuel, diborane also finds valuable use UNITED STATES PATENTS in vulcanizing rubber, as disclosed in United States Pat ent No. 2,558,559. 2,543,511 Schlesinger et al. ------Feb. 27, 1951 I claim: 1. A method of producing diborane which comprises FOREIGN PATENTS reacting a protolyzing acid selected from the group con 451,359 Great Britain ------Aug. 4, 1936 sisting of concentrated chlorosulfonic, concentrated sul furic acid and mixtures thereof with a metal borohydride OTHER REFERENCES selected from the group consisting of the alkali and Kilpatrick et al.: "). Am. Chem. Soc.," vol. 72, pages alkaline earth metal borohydrides in the presence of a 5474-5476 (1950). reaction modifier Selected from the group consisting of Hurd: “Chemistry of the Hydrides,' page 162 (1952), the phosphoric acids, fluorinated phosphoric acids, boric published by John Wiley & Sons, New York, N.Y. acids, the alkali metal salts and the alkyl esters thereof. Schechter et al.: "Boron Hydrides and Related Con 2. The method of claim 1 wherein said modifier is 5 5 pounds," pages 21, 44, Jan. 8, 1951, declassified Jan 5, present in an amount of from about 2% to about 10% 1954, Dept. of the Navy, Bureau of Aeronautics. by weight of the protolyzing acid. Stock: "Hydrides of Boron and Silicon,” 1933, pages 3. The method of claim 1 wherein the said modifier 40-42. is monofluorophosphoric acid. Weberg et al.: "Zeitschrift für Naturforschung,” vol. 4. The method of claim 1 wherein the said modifier is 60 7b, pages 58-59 (1952). orthophosphoric acid. Schechter et al.: "Boron Hydrides and Related Com 5. The method of claim 1 wherein the said modifier pounds,' prepared under Contract NO2(s) 10992 for is dibasic sodium phosphate. Dept. of Navy, Bureau of Aeronautics, prepared by 6. The method of claim 1 wherein the said modifier Callery Chemical Co., printed March 1951, declassified is boric acid. 65 Dec. 1953, page 21.