3,431,087 United States Patent Office Patented Mar. 4, 1969 2 does not require complicated reaction apparatus nor the 3.431,087 prior preparation of either nitryl fluoride or nitrogen pent METHOD FOR THE REPARATION OF HIGH oxide. PURITY NITRONUM SALTS Stephen J. Kuhn, Sarnia, Ontario, Canada, assignor to These and other objects and advantages will become The Dow Chemical Company, Midland, Mich., a cor apparent from the detailed description for the method of poration of Delaware the instant invention presented hereinafter. No Drawing. Filed Feb. 23, 1962, Ser. No. 175,299 In accordance with the method of the present inven U.S. C. 23-356 7 Claims tion, a Lewis acid substance, and particularly a fluoride Int, C. C01b. 21/20 acid substance is introduced into a solution of a nitric 10 acid ester and anhydrous hydrogen fluoride or into a so This invention is concerned with nitronium salts and lution of these compounds carried in nitromethane and more particularly is concerned with a new method for the reaction mixture is agitated while being maintained the preparation of high purity solid nitronium salts which at a temperature ranging from about the freezing point are substantially free from impurities. of the mixture to about 20° above zero centigrade. The The existence and characteristics of solid nitronium salts 5 resulting nitronium salt product, which precipitates dur such as nitronium tetrafluoroborate, nitronium hexafluo ing the course of the reaction, then is separated from rophosphate, dinitronium hexafluorosilicate and nitroni the reaction mixture. The term "fluoride acid substance' um hexafluoroantimonate for example, have been known or "fluoride acid' as used herein refers to those fluorine for sorine time. containing materials capable of accepting a pair of elec These salts at present ordinarily are prepared by one 20 trons in the formation of a bond as set forth by the of the following techniques: (1) nitrogen peroxide Lewis theory of acids and bases. (NO) together with bromine trifluoride (BrF) acts on Ordinarily, in the instant method, the nitric acid ester a suitable material such as a metal, oxide, oxy-salt or and hydrogen fluoride will be carried in a nitromethane halide which is capable of reacting on the bromine tri solvent. However, use of this latter compound is not es fluoride either as a Lewis acid or base Woolf and Emel 25 sential as an excess of the anhydrous hydrogen fluoride eus, J. Chem. Soc. 1050 (1950); (2) preparation of itself can be used as a solvent for the reactants. nitryl fluoride (NOF) and subsequent reaction of this Preferably, as indicated hereinbefore, the Lewis acid compound with a suitable Lewis acid, e.g., a non-metallic substance which reacts with the nitronium ion to form the fluoride Aynsley, Heterington and Robinson, J. Chem. salt will be a fluoride compound and ordinarily will be Soc. 119 (1954); and, (3) adding a mixture of anhydrous 30 a polyfluoride compound of general formula MF, where hydrofluoric acid and an appropriate fluoride compound in the second member M is a metal, metalloid or non to a preparation of dinitrogen pentoxide (NOs) dis metallic species selected from groups III to VII inclusive solved in nitromethaneSchmeisser and Elisher, Z. Natur of the periodic table and n is an integer equal to the forsch., 7b, 583 (1952). valence state of the substance M in the polyfluoride. Ex All of these listed processes for the preparation of 35 amples of a few useful Lewis acid nitronium salt form solid nitronium salts suffer from one or more of the fol ers are: boron trifluoride (BF), phosphorus penta lowing defects and/or difficulties; the operations are mul fluoride (PF5), silicon tetrafluoride (SiF) and the like. ti-step; nitrogen oxide based starting materials, e.g. NO5 Nitric acid esters used in the process are well charac and NOF are both expensive and not readily commer terized uniform compounds unlike nitric acid, which al cially available; the resulting nitronium salts are rela 40 ways has some nitrogen oxide dissolved therein, and read tively impure; and, complicated reactor equipment is ily are available in high purity. The selection of a nitric needed and the reactions are both difficult and somewhat acid ester reactant is not critical or limited. Ordinarily, dangerous to carry out. alkyl nitric acid esters corresponding to the general for Applicant recently has disclosed in a copending appli mula RONO2, wherein R represents an alkyl group hav cation Ser. No. 139,374, filed Sept. 20, 1961, a new, safe 45 ing from 1 to about 5 carbon atoms are used since these and straight-forward method for producing these salts by are commercially available in high purity. Preferably ethyl introducing, in certain specific proportions, a fluorine con nitrate (CHONO2) is employed as the nitronium ion taining Lewis acid substance into a solution of nitric acid Source in the present process. and substantially anhydrous fluoride while maintaining The anhydrous hydrogen fluoride to be used preferably the temperature of the reaction mixture between about is selected from those substantially anhydrous materials 20° C. and the freezing point of the mixture. In the proc ess of this copending application, the nitronium salt prod which assay from about 98 to about 100 percent hydrogen uct precipitates during the course of the reaction and fluoride. readily can be separated from the reaction mixture. The In actual operation of the process, the relative propor salt product, as produced by the method disclosed in this 5 5 tion of nitric acid ester to anhydrous hydrogen fluoride copending application is relatively pure, but contains in the initial solution ranges from about 1/1 to about 2/1 some nitrozonium salt impurities resulting from the pres on a molar basis. Ordinarily, reaction mixtures utilizing ence of nitric oxide impurities always present in the nitric about stoichiometric quantities of hydrogen fluoride and acid reactant. nitric acid ester will be employed as either an excess of It is a principal object of the present invention, there 60 hydrogen fluoride or the nitric acid ester dissolves a por fore, to provide a new and novel process for the prepa tion of the product in the reaction mixture thereby reduc ration of high purity solid nitronium salts free of im ing the product yield. The ratio of the Lewis acid to the purities and particularly free from nitrozonium salt im nitric acid ester ranges from about 1 to about 3 times the purities. stoichiometric molar quantities needed for salt produc It is an additional object of the present invention to tion. Preferably, from about 1.5 to about 2.0 molar equiv provide a safe, straight-forward one step method for the alents of the Lewis acid per molar equivalent of the nitric preparation of extremely high purity, solid nitronium acid ester, will be employed. This excess quantity of Lewis salts which permits the use of readily available commer acid is desired since a portion of the Lewis acid reacts cial materials as reactants. with water formed during the reaction. It is a further object of the present invention to pro The amount of either nitromethane or excess hydrogen vide a method for preparing solid nitronium salts which fluoride solvent to be used is not critical. Generally, the 3,431,087 3 4. amount of solvent utilized will be up to about 150 grams forth in Example 1, 0.75 mole of phosphorus pentafluo per mole of the reactants present. ride (PF) was added to a mixture of 0.5 mole of ethyl The upper operative limit of temperature is the boiling nitrate and 0.25 mole of anhydrous hydrogen fluoride point of the hydrogen fluoride, about 20 centigrade at dissolved in 60 grams of nitromethane. In this prepara atmospheric pressure, and the lower limit is the freezing tion low-boiling Freon 113 (a trademarked product of point of the reaction mixture, i.e. from about 30 to about E. I. duPont Co.) was used as a wash. This facilitated 40° below zero centigrade. A preferred operating tem the subsequent drying of the salt since the fluorinated perature range is from about 20° below zero to about hydrocarbon has a lower boiling point (47.5 C.) than 15° above zero centigrade. nitromethane, normal boiling point (101° C.). In a num The reaction time is not critical as the nitronium salt ber of runs, product yields of from 80 to 90% nitronium precipitates almost instantaneously as the Lewis acid con 10 hexafluorophosphate (NOPF) were obtained, and tacts the nitric acid ester-hydrogen fluoride mixture. A chemical analysis of the salt product for nitrogen gave a smooth reaction which gives control of a preselected oper value identical with that calculated for pure nitronium ating temperature is obtained if the Lewis acid is intro hexafluorophosphate. duced slowly and controllably into the nitric acid ester EXAMPLE 3 hydrogen fluoride mixture while the entire reaction mass is being agitated. Such agitation can be carried out using One mole of silicon tetrafluoride was added to a solu manual, mechanical, electrical or magnetic stirring or by tion containing 1 mole of ethyl nitrate and 1 mole of other conventional mixing techniques. Substantially anhydrous hydrogen fluoride acid in about The process can be carried out in reactor vessels or 100 grams of nitromethane according to the procedure flasks of silica, polyethylene, stainless steel or other mate described in Example 1. rials which do not undergo a prohibitive amount of cor A white solid dinitronium hexafluorosilicate salt rosive attack in the presence of the reactants. Control of (NOat SiF6)] which analyzed to have a nitrogen the reaction temperature within the desired limits can be content the same as calculated for the pure product, was achieved through the use of a reactor with integral cool 2 5 produced. ing means or by coupling an external cooling means to EXAMPLE 4 a given reactor. A number of runs were made reacting borontrifluoride, The nitronium salts as produced are white solids and ethyl nitrate and hydrogen fluoride in nitromethane ac are of a very high purity. These can be removed from the cording to the procedure described in Example 1. In this reaction mixture by conventional means such as filtration, 30 study, the ratio of BF3 to CHONO was varied for each centrifugation and the like, and then dried directly. How test. ever, if desired, the separated salts can be washed with Table 1 which follows shows the effect of the a small amount of nitromethane and any residual wash material then simply be removed under reduced pressure. BF CH5ONO Also low boiling inert diluents such as certain of the 3 5 ratio on product yield. Freons (trademarked product of E. I. du Pont Co.), meth ylene chloride, chloroform and the like can be used as TABLE 1. Reactants (moles) NO+BF a wash to facilitate the subsequent drying of the salts. Run No. Solvent ( 6." ) The salts find use as oxidizers in high energy fuels. BF C2H5ONO HF (grams) yield, Additionally, these can be used as nitrating agents for 40 percent aromatic organic compounds especially if anhydrous me 0.75 0.5 0.5 60 74.9 0.875 0.5 0.5 60 84.2 dia are required for the nitrations. Also, they may be .00 0.5 0.5 60 93.4 used as oxidizers in a number of reactions replacing the 1.25 0.5 0.5 60 91.0 conventional oxidizers now being used. The high purity products produced by the instant method are especially The nitronium tetrafluoroborate product from each of suitable for kinetic investigations or for any process or these runs upon analysis was found to be of the same utility requiring extremely high purity nitronium salts. high purity as the products prepared in the preceding The following examples will serve to further illustrate examples. the method of the present invention but are not meant to In a manner similar to that described for the fore limit it thereto. 50 going experiments, nitronium tetrafluoroborate can be EXAMPLE 1. produced by reacting substantially stoichiometric quan tities of borontrifluoride with butyl nitrate in the pres Into a solution comprised of about 0.5 mole of ethyl ence of excess hydrogen fluoride at a temperature of nitrate and about 0.5 mole of substantially anhydrous about 10' C. Nitronium hexafluorophosphate can be hydrogen fluoride in about 60 grams of nitromethane produced using methyl nitrate-anhydrous hydrogen fluo there was added slowly with stirring about 1 mole of ride reactants in nitromethane solvent and adding a molar gaseous boron trifluoride, the reaction temperature being excess, based on stoichiometry, of phosphorus penta maintained at from about minus 20 to about positive 15 fluoride thereto at a temperature of about minus 10° centigrade. As this addition was being made, a white centigrade. Also, nitronium hexafluorosilicate can be solid precipitate of nitronium tetrafluoroborate 60 formed by adding silicon tetrafluoride to a solution of amyl nitrate and anhydrous hydrogen fluoride. precipitated in the reaction flask. After the boron trifluo Various modifications can be made in the present in ride addition was complete, the stirring was continued for vention without departing from the spirit or scope thereof a short while. The white solid precipitate than was fil for it is understood that I limit myself only as defined tered from the reaction solution and the separated salt in the appended claims. washed twice with two 25 milliliter portions of nitro I claim: methane. This product was dried by removing the wash 1. A process for preparing high purity nitronium salts material under reduced pressure. The product yield, based Substantially free from nitrozonium salt impurities which on the amount of ethyl nitrate reactant, was about 93.4%. comprises: introducing a fluorine containing Lewis acid Chemical analysis of the salt showed a nitrogen con Substance into a solution of a nitric acid ester and sub tent of 10.5 percent. Calculated nitrogen value for the stantially anhydrous hydrogen fluoride while maintain pure salt is 10.5 percent. ing the temperature of the reaction mixture between about 20 C. and the freezing point of the mixture, said Lewis EXAMPLE 2 acid substance being a member selected from the group Using the same technique and procedural steps as set consisting of boron trifluoride, silicon tetrafluoride and 3,431,087 5 6 phosphorus pentafluoride and said nitric acid ester being maintaining the reaction temperature between about 20 an alkyl nitric acid ester corresponding to the general below zero and 15 above zero centigrade, the molar ratio formula RONO where R represents an alkyl group hav of said boron trifluoride:ethyl nitrate:hydrogen fluoride ing from 1 to about 5 carbon atoms, the relative pro in the reaction mixture being about 2:1:1, and, separat portion of said nitric acid ester and substantially anhy ing the resulting solid high purity nitronium tetrafluoro drous hydrogen fluoride in the initial reaction solution borate substantially free from nitrozonium salt impuri ranging from about 1/1 to about 2/1 on a molar basis ties from the reaction mixture. and the ratio of said Lewis acid substance to said nitric 6. A process for preparing high purity nitronium hexa acid ester ranging from about 1 to about 3 of the stoichi fluorophosphate substantially free from nitrozonium salt ometric molar quantity needed for salt production there O impurities which comprises: introducing phosphorus by to precipitate directly in said reaction mixture the pentafluoride into a solution of ethyl nitrate and sub corresponding high purity nitronium salt substantially stantially anhydrous hydrogen fluoride dissolved in nitro free from nitrozonium salt impurities. methane while maintaining the reaction temperature be 2. The process as defined in claim wherein there is tween about 20 below zero and 15 above zero centi incorporated the additional step of separating the nitro grade, the molar ratio of said phosphorus pentafluoride: nium salt from the reaction mixture. ethyl nitrate: hydrogen fluoride in the reaction mixture 3. A process for preparing high purity nitronium salts being about 3:1:2, and, separating the resulting high substantially free from nitrozonium salt impurities which purity nitronium hexafluorophosphate substantially free comprises: introducing a Lewis acid fluoride compound from nitrozonium salt impurities from the reaction into a solution of a nitric acid ester corresponding to 20 mixture. the formula RONO wherein R is an alkyl radical con 7. A process for preparing high purity dinitronium taining from 1 to 5 carbon atoms and substantially an hexafluorosilicate substantially free from nitrozonium hydrous hydrogen fluoride dissolved in a solvent selected Salt impurities which comprises: introducing silicon tetra from the group consisting of nitromethane and excess fluoride into a solution of ethyl nitrate and substantially hydrogen fluoride while maintaining the reaction tem 25 anhydrous hydrogen fluoride dissolved in nitromethane perature between about 20° above zero centigrade and while maintaining the reaction temperature between about 30° below zero centigrade, said Lewis acid fluoride being 20 below zero and 15° above zero centigrade, the molar a member selected from the group consisting of boron ratio of said silicon tetrafluoride: ethyl nitrate: hydrogen trifluoride, silicon tetrafluoride and phosphorus penta fluoride in the reaction mixture being about 1:1:1, and, fluoride, the relative proportion of said nitric acid ester 30 separating the resulting high purity dinitronium hexa to said hydrogen fluoride in the initial reaction solution fluorosilicate substantially free from nitrozonium salt im ranging from about 1/1 to about 2/1 on a molar basis purities from the reaction mixture. and the ratio of said Lewis acid fluoride to said nitric acid ester ranging from about 1.5 to about 2.0 of the References Cited stoichiometric molar quantity needed for production of 35 Taylor et al., "Sidgwick's Organic Chemistry of Nitro said high purity nitronium salt thereby to precipitate gen," 1937, pp. 248-252. directly in said reaction mixture the corresponding high Remy, "Treatise on Inorganic Chemistry,’ 1956, vol purity nitronium salt substantially free from nitrozonium ume I, pp. 599-601, 622. salt impurities. Maddock et al., "Recent Aspects of the Inorganic 4. The process as defined in claim 3 wherein there is 40 Chemistry of Nitrogen,’ 1957, Special Publication No. incorporated the additional step of separating the nitro 10 of The Chemical Society, London, pp. 23-32, nium salt from the reaction mixture. 5. A process for preparing high purity nitronium tetra MILTON WEISSMAN, Primary Examiner. fluoroborate substantially free from nitrozonium salt im purities which comprises: introducing boron trifluoride 45 U.S. C. X.R. into a solution of ethyl nitrate and substantially anhy 23-357, 358 drous hydrogen fluoride dissolved in nitromethane while