Articles Indian Journal of Chemical Technology Vol. 9, May 2002, pp. 223-226 Study on the formation of dinitramide using mixed acid nitrating agents G Santhosh, S Venkatachalam*, M Kanakavel & K N Ninan Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 9 August 2001; revised received 27 February 2002; accepted 21 March 2002 Nitration of ammonium sulphamate was carried out using a mixture of sulphuric acid and nitric acid at -30 to - 40°C. The mole ratio of sulphuric acid to nitric acid was varied from 0 to 4 and the extent of formation of ammonium dinitramide (ADN) was meas:red. The initial yield of ADN increases with increase of sulphuric acid content in the acid mixture and starts decreasing as the reaction time is increased. The variation of product yield with change in reaction time and total acid concentration was studied. There has been a lot of interest in recent years for the Experimental Procedure development of new high energy halogen-free Materials oxidizers like ammonium dinitramide(ADN), Ammonium sulphamate AR (SRL, Bombay) was hydrazinium nitroformate(HNF), hexanitrohexaaza­ powdered well in a mortar and pestle and was further isowurtzitane(CL-20), 1,3,3-trinitroazetidine (TNAZ) dried in vacuum. Conc. H S0 98% (Qualigens, Bombay) was used etc., having high density and heat of formation I. 2 4 as received. Ammonium dinitramide (ADN) is the ammonium salt Fuming HN0 > 98% was distilled in the laboratory of 1,1 ,3,3-tetraoxo-l ,2,3-triazapropene anion. 3 from a mixture of 1: 1 by weight of fuming HN0 Dinitramide salts are useful oxidizers for high energy 3 (92%) with conc. H2S04. The fraction between 83- materials such as propellants, pyrotechnics and gas 8SoC was used. generating formulations. Russians at the Zelinsky Liquor ammonia AR, about 2S% NH3 (Qualigens, Institute of Russia in 1971 invented a method for 2 Bombay) was used as received. synthesizing dinitramide anion . Subsequently Bottaro and co-workers in USA invented methods in 1989 for Methods the synthesis of both inorganic and organic Ammonium sulphamate (S.7 g, O.OS M) was added dinitramides which could provide a new class of in small portions to a mixture containing H2S04 energetic materials. They are generally prepared3-8 by (98%) and fuming HN03 at a temperature range of -3SoC to -4SoC in a 3 necked RB flask with stirring. nitration of deactivated amines like NH2N02 using The rate of addition was controlled in such a way that exotic nitrating agents namely N02BF4 or N20 5. Langlet et al. 9, have reported that dinitramide and its the temperature of the mixture did not exceed -3S°C. salts can be prepared by nitration of ammonia-sulfate The reaction mixture was then poured into about 100 g of crushed ice. The diluted acid was neutralised derivatives such as NH4S03NH2, NH(S03NH4h etc_. by ammonia solution. The pH of the solution was using a mixture of HNOiH2S04. The ability of checked during the course of the neutralisation and dinitramide anion to form stable oxygen rich salts was continued till the solution became alkaline with a variety of cations to make compounds of high (PH -8). densities makes it a promising candidate for the The neutralised solution was diluted to known development of energetic oxidisers in solid concentration and analyzed by UV spectroscopy. The propellants. The present paper gives a detailed yield of the product was calculated by measuring the investigation on the formation of dinitramide using absorbance at 284 nm using the calculated molar mixed acid nitrating agents under different conditions. extinction co-efficient (E). The neutralised solution was then vacuum evaporated and dried. The dried *For correspondence: (Fax 91-471-415236, solids were then extracted with isopropanol, E-mail: valli@sancharneLin) concentrated and dried. This was purified further by Articles Indian 1. Chern. Techno!., May 2002 extraction with ethyl acetate to get ammonium Table I-Product yield at different times dinitramide (ADN). Mole ratio of Mole ratio % ~ield of (!roduct* UV spectrum of the sample was recorded on AS : Total acids of SA/NA 10 min 20min 30min CARY 5E UV-VIS-NIR spectrometer as water 1:8 0:8 14 solution in the range of 200-400 nm. 1:8 2:6 17 26 30 Infrared spectrum of the product was recorded on 1:6 4:2 27.5 45 35 Nicolet 5 lOP FfIR spectrometer in the range of 4000- 1:4 2:1 33 26 34 l based on UV spectroscopy 400cm· . * Thermal study of the product by DSC was carried The acid mixture with SAINA in the mole ratio of 2: I out using DSC-T A Instruments model DSC - 2920 at is very viscous at -40°C and hence fast addition of the a heating rate of 5°C per minute. reactant (AS) at a controlled rate and vigorous stirring Results and Discussions becomes very difficult In the nitration, the first step is the formation of As the molar concentration of the acids increases, the N0 + concentration also would increase nitronium ion (N02+) by ionisation of HN03 by strong 2 correspondingly. When equimolar mixture of acids acids such as HF, HCI04, H2S04 or solid acid catalysts 10. The latter compounds promote ionisation were taken, the product yield increased with increase of the total acid concentration (i.e., sulphuric acid and of HN03 to N02+. H2S04 is one of the most frequently employed reagents during nitrations. nitric acid). An yield of 34% was obtained in 30 min In a mixture of different ratios of HNOiH2S04, the when equimolar mixture of acids were taken. mole percent of N02+ present have been measured It is seen from Table 1. that as the sulphuric acid to and well-documented II. For a given concentration of nitric acid mole ratio increases from 2:2 to 4:2 the yield of the product increases. When the mole ratio of HN03/H 2S04 the maximum concentration of N02+ is obtained when the mole ratio of H2SOJHN03 is about SA to NA is 2:6 the yield of the product decreases. 2. During aromatic nitration using mixed acid This is because the excess acid present in the system nitrating agents it is necessary that the nitronium ion reacts with the dinitramidic acid formed, thereby concentration is as high as possible and the attack of decreasing the yield. When SA/NA mole ratios are the latter with the aromatic substrate is the slow rate I: I and 2: I, the reaction mixture becomes very determining step. In the case of aliphatic nitrations the viscous at temperatures below -30°C. Since the attack of nitronium ion is much faster. For the product yield during nitration depends on important conversion of ammonium sulphamate (AS) to parameters like viscosity of the medium, temperature, dinitramidic acid [HN(N02h], the stoichiometric ratio agitation speed and rate of addition of the reactants, of the nitronium ion to reactant AS is 2 as per Eq. (1). the formation of the dinitramidic acid and its The neutralization step is given in Eq. (2). decomposition in acid medium becomes a critical factor determining the yield of the product. These NH4S03NH2+2N02+ +2HS04- +H 2 0~HN(N0 2h factors can be overcome by increasing the rate of +2H2S04+NH4HS04 --. (l ) agitation of the reaction mixture, cooling the reactor at much faster rate and also by making the system less HN(N02h+2H 2 S0 4 +NH 4 HS0 4 +6NH 3 ~NH4N(N0 2h viscous, so that the transfer of nitronium ion to the +3(NH4hS04 __ . (2) substrate takes place effectively. The viscosity of the medium could be decreased when more nitric acid is Experiments were carried out using different ratios taken in the system. In order to reduce the vi scosity of of nitric acid(NA) and sulphuric acid(SA) for the reaction medium and to promote effective stirring, different period of time. A control experiment was experiments were carried out with sulphuric acid to performed using nitric acid alone and the results were nitric acid in the mole ratio of 1:3 . The product yield given in Table 1. increases as the reaction time is increased. An yield of When experiments were carried out, using the 30% is obtained in 30min for SAINA mole ratio 1:3 stoichiometric ratio of 2, a maximum yield of 45 % of and 14% for SAINA mole ratio 0:4. the product was obtained in 20 min, and as the Experiments were carried out for different mole reaction time was increased to 30 min, the product ratios of sulphuric acid to nitric acid (from 0 to 4) and yield decreased to 35% due to the decomposition of the yields of the product obtained for different periods the product with excess acid present in the system. of time are shown in Fig. 1. 224 Santhosh et at.: Formation of Dinitramide Using Mixed Acid Nitrating Agents Articles Fig. 1. shows the product yield plotted against the Ammonium dinitramide shows UV maximum in l2 reaction time for the reactions carried out with mole water at 212 and 284 nm . UV spectrum of ratios of SAIN A 1 :3, 1: 1 and 2: 1. From the figure it is ammonium dinitramide in water is given in Fig. 2. seen that when the sulphuric acid to nitric acid mole The absorbance values at 284 nm were measured for ratio is 2: I the product yield reaches a maximum at solutions of different concentrations and plotted 20 min. When the mole ratio is 1: 1 it reaches a against the concentration. The extinction coefficient maximum at 10 min.