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 25 produced.