Assessment of the synthesis routes conditions for obtaining ammonium dinitramide by the FTIR

Over the last two decades, many routes have been proposed to synthesize ammonium dinitramide (ADN). However, most of them lie in routes in which reactants are too expensive for large-scale production. In this sense, the use of ordinary reactants is of paramount importance in this case. The aim in this synthesis consists on nitrating a starting reactant in a reaction known as nitration. Both the nitrating agent and the starting reactant should preferably be ordinary, narrowing the possibility of having realistic options for them. The most ordinary nitrating agent consists of a mixture of sulfuric and fuming nitric acids. Therefore, the breakthrough must come from the suitable choice of the starting reactant. However, so far, the only viable reaction relies on the use of sulfamate salts. Even though the process with this kind of salt has been largely commercially developed, only few information are available in the literature to properly address issues emerged from it. In this study, an attempt is made to enlighten some effects on the product caused by modifications in the route conditions. Characterization of the resulting products was confirmed by FT-IR in the region of MIR and NIR. The characteristic bands employed to identify ADN in the region of middle infrared were: 3129 and 1384 cm-1 (NH4); 1537, 1344, 1209, and 1177 cm-1 (NO2); 1032, 954 cm-1 (N3); 828, 762 and 732 cm(NO2). The near infrared analysis pointed out few bands at 5185 and 4672 cm-1 in NH combination bands region. The resulting middle infrared spectrum was compared to the reference found in the literature for this product. The results show excellent agreement with the expected product.


INTRODUCTION
Solid propellants are mainly composed of particulates of energetic materials.The study of these energetic materials is the way to optimize performance, burning behavior, stability, detonation properties, processing characteristics, and mainly their sensitivity (Teipel, 2005).Several compounds can take part in solid propellant composition, and although ammonium perchlorate (AP) is the most cited in the literature among the oxidizers, its downside, however, lies in the fact that it can release a large amount of chloride into the environment (Nagamachi et al., 2009).
Based on the chemical structure, CL-20 or hexanitrohexaazaisowurtizitane (HNIW) and ammonium dinitramide (ADN) are potential components to be used in solid propellant formulation due to their special characteristics.Even though performance is the primarily criterion for the designer to define the convenience of any material, there are other practical considerations, including: availability and price; thermal and mechanical sensitivity (insensitive ammunitions -IM); processibility; compatibility; chemical and thermal stability; mechanical behavior temperature dependence; burning behavior; and pressure exponent (Teipel, 2005).It is also crucial to consider the oxidizing properties (including the oxygen positive balance) in the propellant development process.ADN has positive oxygen balance of +25.8%, being a promising energetic material as a viable alternative to replace the ammonium perchlorate (AP) as an oxidizer in solid propellants.Its application can be found in civilian rockets and in smoke-low-emitting missiles (Teipel, 2005;Wingborg, 2010).
Monopropellant for micro-propulsion systems for space applications are almost based on hydrazine.This compound, however, is highly toxic, volatile, and carcinogenic.Therefore, it demands high costs in safety measures at the whole process.Hence, over the years, there has been a considerable interest in Europe and in the USA in finding possible substitutes for hydrazine.ADN liquid monopropellants seem to be also a potential substitute to replace it whether for being easier to handle or for its specific impulse, 10% higher than for hydrazine (Wingborg, 2005).ADN was cited for the first time by professor Tartakowsky at the Annual Conference (ICT), in 1993 (Nagamachi et al., 2009).
Simultaneously, in the USA, it was presented as a new oxidizer for rocket-solid propellants by Pak (Teipel, 2005).
ADN has the chemical formula NH 4 N(NO 2 ) 2 (Squeme 1), which is of primary interest in solid-rocket propellants as a potential substitute for AP for keeping high efficiency in systems that already employ AP.The use of ADN in combination with an energetic binder can enhance the specific impulse (I sp ) to reach values around 2,600 Ns/kg.If a metal fuel, such as Al or AlH 3 , is also incorporated to this formulation, the specific impulse can reach even higher, around 2,700 to 2,800 Ns/kg (Teipel, 2005).the decomposition of the dinitramide.This synthesis is completed by recovering ADN from the reaction medium followed by purification.This process was developed by FOI and Nexplo and now it has been used for commercial production (Teipel, 2005).
Several techniques are cited in the literature for characterization of ADN (Christe et al., 1996;Santhosh et al., 2002).Christe et al. (1996) have used Raman spectroscopy and infrared, in the middle infrared (MIR) region, to characterize ADN.The infrared (IR) bands attribution were: 1526 cm -1 , attributed to the unsymmetrical stretching (υ a ) in phase of group NO 2 ; 1344 cm -1 of symmetrical stretching (υ s ) in phase of group NO 2 ; and 1181 cm -1 (υ s out of phase of group NO 2 ).Santhosh et al. (2002) have used ultraviolet spectroscopy (UV), IR, and thermal methods and they have observed that the solution of ADN presents strong absorption with maximum at 284 nm under UV, and that IR spectra showed characteristic bands at 3136 cm -1 (υ N-H of NH 4 + ), 1531 cm -1 (υ a in phase of group NO 2 ), and 1344 cm -1 (symmetrical stretching (υ s ) in phase of group NO 2 ).
In this paper, ammonium sulfamate and sulfamic acid were used as starting materials for obtaining ADN in different route conditions for these syntheses.The proportion of reactants and purification methods were tested, and the resulting samples were analyzed in the MIR region.For the synthesis in which ammonium sulfamate was used as a starting reactant, the samples were also analyzed by near infrared (NIR) spectroscopy.Thus, it was expected to cover a more ample zone in the spectral region of IR, which is meant for a suitable characterization of the resulting products.

Routes of synthesis for obtaining ADN
The routes applied in this study make use of sulfamic acid and ammonium sulfamate as starting reactants.The route conditions for these syntheses were adapted from Vörde and Skifs (2005) and Langlet, Ostmark and Wingborg (1997).(Shaw, 1993).
Among all the possible routes for this synthesis, in this paper, it was focused on the nitration of amidesulphonates.On one hand, the reaction can be carried out in the absence of organic solvents (Squeme 2).However, on the other hand, it must take place in a strong acid medium, which can also lead to Squeme 2. Reaction involved in the route of ADN synthesis, by nitration of amidesulphonates (Teipel, 2005).

Nitration
For nitration of sulfamic acid, 40 mL of fuming nitric acid (96%) and 20 mL of sulfuric acid (95%) were mixed in a 500 mL 3-neck-round flask.The mixture was cooled down to -37ºC in a cryogenic bath (Ultra Termostato Criostato of Optherm, with controller HAAKE DC3).It was used 40% CaCl 2 in water as the refrigerant fluid.Sulfamic acid was added in small portions of 0.5 to 1.0 g (total=5 g) under strong agitation (250 rpm) with a mechanical stirrer, the mixture was left to react for 1.5 hour at the temperature ranging from -30 to -35ºC.

Neutralization
The reaction was conducted for 1.5 hour, then the whole mixture was transferred to a 1 L erlenmeyer, containing 85g small pieces of ice (deionized water) and equipped with magnetic stirrer.The erlenmeyer, was immersed and surrounded by pieces of ice inside a large flask.The aim of this procedure was to dilute the mixture before adding base, which can prevent it from reacting strongly with the acid mixture.Ammonium hydroxide was slowly dropped with a 200 mL burette, aiming at preventing the temperature inside the mixture from surpassing 0º C. The pH of solution was measured with litmus.Ammonium hydroxide was added until the solution became slightly alkaline.It was used 142.5mL of base for the neutralization.

purification
The mixture provided from the neutralization step was transferred to a three-neck-round flask, and the liquid was left to vaporize in a bath at 35°C under vacuum (maximum capacity).After 80% of the liquid was vaporized, the round flask was removed and, with the aid of a spatula, the precipitated stuck on the flask was scratched out.This material was slightly ground and left inside the round flask.Acetone was added to promote dissolution of ADN.The round flask was manually shaken several times for 15 minutes.The content in the round flask was filtered with a paperfilter funnel, which was transferred to an erlenmeyer and newly cooled in cryostatic bath.White precipitated was formed again, which was finally analyzed by the IR.This cooling and filtering process was repeated for three times.Every filtered collected with acetone was placed in a warm bath and kept at nearly -33ºC under intense vacuum until the whole acetone was vaporized.The left liquid was a yellow solution highly concentrated.This solution was then transferred to a watch glass and left to vaporize at ambient temperature.

Nitration
Nitration has been conducted at temperatures ranging from -35 to -37º C in all the experiments.Concentration of acids, time of reaction, and mass of ammonium sulfamate have been varied, according to Table 1.

Neutralization
The resulting mixtures from the nitration were diluted and cooled with 100g of ice, except ADN 40F/20 (40mL HNO 3 Fum/20mL H 2 SO 4 ) with "no dilution", in which this step was eliminated.The mixtures were neutralized with 70 to 140mL of ammonium hydroxide.400 cm -1 , resolution 4 cm -1 and gain 1.The samples were prepared according to the trasmission technique, with KBr pellets with concentrations around 0.5: 400 mg.For the NIR analysis, the samples were analyzed with the same equipment in the region ranging from 7800 to 4000 cm -1 , under the same conditions.

RESULTS AND DISCUSSION
MIR analysis of the starting material used in the route 1 Squeme 3 shows the chemical structure of the starting material used in the referred route of synthesis manually for 10 or 45 minutes, respectively in each case.
The mixture was filtered in a Buchner funnel and washed with approximately 300 mL of deionized water (Santhosh et al., 2002).Desorption of ADN was carried out by recirculating acetone through a Soxhlet, in which GAC was covered by a paper-filter.Vacuum and temperature were controlled in order to keep it at 60° or 50° C, respectively, in each case.The rest of the material inside the round flak was analyzed by IR.
For the synthesis of ADN 40F/20 with "no dilution", ADN 35F/35F and ADN 40F/20, the neutralized solution was filtered and the precipitated was washed with acetone.
The resulting liquid turned out yellow.A large amount of precipitated lying on the filter was discarded.Water formed during the reaction and acetone in the filtered were removed at 40º C under vacuum.The purification process was repeated for three times, and the same yellow product was obtained each time.This product was left on a watch glass inside a desiccator under vacuum, and then it was analyzed by IR.

Analysis by IR
Spectrum one spectophotometer of PerkinElmer had been used in this paper, in the region ranging from 4000 to  1181, 828 and 738 cm -1 (NO 2 ); 1025 and 954 cm -1 (N 3 ).However, in the MIR spectrum (Fig. 2A), only one band at 1380 cm -1 can be pointed out, which situates in the region characteristic of Group NH 4 + of ADN.The MIR spectrum of the resulting sample is more similar to ammonium nitrate (Fig. 2B), indicating that, according to this route, this product is basically ammonium nitrate, which is likely obtained owing to the degradation during the purification process of the synthesis product.

MIR analysis of the starting material of route 2
Squeme 4 shows the chemical structure of the starting material used in this route.
MIR analysis of the resulting product in route 2 (using ammonium sulfamate): ADN 38/12 (38 mL HNO 3 /12 mL H 2 SO 4 ) Like route 1, in route 2 any characteristic bands of ADN were found.Instead, only one band at 1380 cm -1 , characteristic of the group NH 4 + , was found.The spectrum of the resulting product was more similar to ammonium nitrate, which presents large bands.
NIR analysis was conducted in this case since MIR characteristics bands of ADN were observed, which allows a more ample characterization of the resulting product (Fig. 7).Some number of bands at 5185 and 4672 cm -1 were seen.They were situated in the combination region of composed bands that present groups NH (Goddu, 1960).

CONCLUSION
Analysis conducted by FT-IR spectroscopy has proved a very effective way to characterize ADN, starting reactants, and sub-products.All characteristic bands for these compounds could be found, besides the resulting spectra, which also allow concluding that ADN 40F/20 (40 mL HNO 3 Fuming/20 mL H 2 SO 4 ) route is the most promising to synthesize ADN among the alternatives tested in this paper.The result was only concluded based on IR spectroscopy.However, more researches must be done in order to learn about purity, yield, and how to deal with the large amount of sub-products.

Figure 1 Figure 2
Figure 1 presents the sulfamic acid MIR spectrum, whose bands are at the same wave numbers found in the spectrum from the reference in the literature