Chapter 3

Synthesis and Characterization of Dinitramide Salts

Part of the results from this chapter has been published:

1. G. Santhosh, S. Venkatachalarn, M. Kanakavel, K.N. Ninan "Study On The Formation of Dinitram~de Using Mixed Acid Nitrating Agents" Indian Journal of Chemical Technology. 2002, 9. 223-226.

INITRAMIDE anion having the structural formula - N ( N 0 2 ) ? forms

new class of salts with a wide variety of cations. The ammonium

salt of this anion, ammonium dinitramide (ADN), is a candidate

oxidizer for solid propellants and it considerably exceeds ammonium

perchlorate in propellant performance. The acid form of this anion,

dinitramidic acid (DNA, HIV(N02)2) is one of the strongest inorganic acids

and hundreds of simple and complex salts can be synthesized on the basis

of reactions of well-known inorganic acids. There are different approaches

to the synthesis of dinitramide salts as reviewed in part 2 of chapter 1. The

focus of the present chapter is to explore in detail the chemistry of the

formation of dinltramide on the nitration of deactivated amines using mixed

acids. This chapter is divided into two parts. The first part deals with the

synthesis and elaborate studies on the formation of ADN and its

characterization using various analytical methods. The effect of variation of

ratio of reactants on the yield of ADN is explored in detail. The studies lead

to the selection of an optirnum ratio of reactants for maximum yield of ADN.

The second part of the chapter details the synthesis of few more

dinitramide analogues viz., potassium dinitramide (KDN), guanylurea

dinitramide (GUDN), tetramine Cu(ll) dinitramide and their characterization

by spectral and thermal methods. It also discusses a process for making

coarse ADN grains from as-synthesised ADN.

3.1. Synthesis and Characterization of Ammonium Dinitramide (ADN)

ADN is the ammorlium salt of 1 , I ,3,3-tetraoxo-I ,2,3-triazapropene

anion [ ' I . D~nitramide salts are useful oxidizers for high-energy materials

such as propellants, pyrotechnics and gas generating formulations [2,31. The

syntheses of dinitramide salts by various methods have been reported by

several authors 14-'21. Majority of the synthetic methods on the synthesis of

ADN are from patented literature. The dinitramide salt is generally prepared

by nitration of deactivated amines viz., NH2N02. NH2COONH4,

NH2COOC2H5, NH3 etc.. using very strong nitrating agents namely N02BF4

or N2O5 14-91. The ability of dinitramide anion to form stable oxygen rich salts

with a variety of cations to make compounds of high densities makes it a

promising candidate for the development of high performance solid

propellants. The present chapter discusses the low temperature nitration of

ammonium sulphamate (AS, NH2S03NH4) using mixed acids (HN03 &

H2S04) and the mechanism of formation of ADN.

This part of the chapter describes the synthesis, characterization and

the influence of reaction parameters like the variation of reactant ratios on

the formation of arnrnonium dinitramide. The effect of sulphuric acid and

nitric acid ratio in the mixed acid on the yield of dinitramide is studied in

detail. The chapter also highlights the kinetics and mechanism of the

nitration reaction and the influence of added water molecules on the

dinitramidic acid for ma ti or^. The separation of ADN by solvent extraction

and adsorption is highlighted. Characterization of ADN by spectroscopic

and therrnoanalytical methods is also described.

3.2. Experimental

3.2.1. Materials

Ammonium sulphamate AR (SRL, Bombay) was powdered in a

relative humidity of 50% and was further dried in vacuum to obtain a free

flowing powder (m.p 130-132°C). Con. HzS04 98% (Qualigens, Mumbai)

was used as received. Fuming HN03 of assay > 98% was distilled in the

laboratory trom a mixture of 1:l (by weight) of fuming HN03 (92% ) with

con. H2S04. The fraction between 83 - 85°C was used. Liquor ammonia

AR, about 25% NH3 (Qualigens, Mumbai) was used as received. The

solvents isopropanol and ethylacetate were distilled and dried over

molecular sieves prior to use.

3.2.2. Synthesis of Ammonium Dinitramide (ADN)

I n a typical experirrlent ammonium sulphamate (5.79, 0.05mol) was

added in s~nall portions to a mixture containing HN03 (assay-98%,18.9g,

0.3mol) and con.H2S04 1(9.89, 0.lmol) maintained at a temperature of

-35" C to -45" C in a 3-necked RB flask with stirring. The rate of addition

of AS was controlled in such a way that the temperature of the mixture

does not exceed --35" C. Formation of white precipitate was observed

during the course of the reaction and the viscosity of the mixture increases

as the reaction progresses. Stirring was continued for pre determined time

intervals (1 0 minutes to 40 minutes). The reaction mixture was then diluted

by pouring into about 1009 of crushed ice. The diluted acid solution was

neutralized immediately by addition of cold liquor ammonia solution while

maintaining the temperature below 0°C. The pH of the solution was

checked during the course of the neutralisation and it was continued till the

solution becomes slightly alkaline (pH - 7.5 to 8).

The dinitram~dic acid (DNA) formation (section 3.3.1) was measured

by taking samples from the reaction mixture at regular intervals and then

measuring the absorbance at 284nm. Figure 5.1 shows a typical case of

increase in absorbance wii.h time obtained during the nitration reaction.

Figure 5.1: Change in absorbance during the course of reaction

Figure 5.1 shows an increase in absorption at 284nm (characteristic

of -N (N02 )~ ) for the samples taken at different time intervals. However the

absorption curve at 40min showed a decrease in absorbance at 284

indicating the optimum time is achieved for the maximum conversion of

dinitramidic ac~d is betweell 30 and 40min. The reaction is discontinued at

this moment and further processed. Small weighed quantity of the

neutralized solution was diluted to a known concentration and analyzed by

UV spectroscopy. The yield of ADN was calculated by measuring the

absorbance at 284nm using the experimentally determined molar extinction

co-efficient as explained in chapter 2. section 2.6.

3.2.3. Separation of Ammonium Dinitramide

Two methods of separation of ADN from aqueous solution were

practiced. In the first method, the neutralized solution was evaporated in a

rotary evaporator under vacuum to completely remove water. The obtained

solid was further drled under vacuum. The dried solid was then extracted

with hot isopropanoi (500rnl) in batches, and the isopropanol extracts were

flltered through a filter paper, concentrated under vacuum and dried to give

a crystalline solid. The obtained solid was purified further by extraction with

hot ethyl acetate (200ml), filtered and evaporated under vacuum to yield

pale yellow crystals of pure ADN. The yield is 70%.

The second methotj of separation involves the use of different

adsorbents to selectively adsorb ADN. Different adsorbents are explored

for the effectiveness in adsorbing ADN. Adsorbents like activated charcoal,

granulated charcoal, silica gel, alumina etc., have ability to adsorb ADN

from aqueous solutions. The adsorbed ADN was then eluted using

solvents such as hot water, methanol, acetone etc., and then evaporated

under vacuum to obtain 23 free flowing pale yellow powder. A detailed

investigation on the use of powdered and granulated charcoal for the

adsorption of ADN from aqueous solutions was carried out and the results

are given in Chapter 4 (vlde supra).

3.3. Results and Discussion

3.3.1. Nitration of Ammonium Sulphamate (AS) using Mixed Acids

In the nitration, the first step is the formation of nitronium ion (N02' )

by ionization of HN03 by s'trong acids such as HF, HC104 , H2S04 or solid

acid catalysts 'I". The latter compounds promote ionization of HN03 to

NO>'. H2S04 IS one of the most frequently employed reagents during

nitrations. The sequence of reactions occurring with HzS04 and HN03 is

given in Equat~ons 3.1 to 3.4.

2 HN0,--, NO,'+ NO,. + H 2 0

H2S04+ H 2 0 - H,O++ HSO;

HNO, + H,>O - ---+ H,O'+ NO,.

In a mixture of different ratios of H2S04/HN03, the mole % of NO2'

has been measured and well documented L'411. For a given concentration of

H2S04/HN03, maximum cc~ncentration of NOz' is obtained when the mole

ratio of H2S04 IHNO:{ is 2. During aromatic nitration using mixed acids it is

necessary that the nitronium ion concentration is as high as possible. The

attack of the latter with the aromatic substrate is the slow rate-determining

step. For the conversion of AS to dinitramidic acid (DNA) the required

stoichiometric ratio of the nitronium ion to AS is 2 as per Equation 3.5.

NH4S0,NH, + 2 NO,' + 2 HSO; + HzO - HN(N02), + 2 H2S04 + NH4HS04

3.3.2. Derivation of a Reaction Scheme

Unlike aromatic nitration, in the present case mono nitration on the

nitrogen of the substituted amine [A] to mono nitramine [B] followed by

further nitration of the latter to the dinitramine [Dl or the nitration of

nitramide [C] has to take place as shown in scheme 3.1. The formed mono

nitro interrned~ate should encounter another NO>' without undergoing

decomposition The mechanism of formation of mono and dinitramine can

be represented by the reaction scheme given in 3.1.

' NH,SO,NH; + NO,' H,N+-S03NH,

Scheme 3.1: Mechanism of formation of mono and dinitramide

The formation of dinitramidic acid is suggested as a network of

parallel and consecutive reactions. The dinitramidic acid is formed via two

intermediates, which are able to react in different manners. Apart from the

formation of nitramide [C] and dinitramidic acid, their decomposition should

also be considered as shown in Equations 3.6 and 3.7. The

autocatalyt~c decomposition of dinitramidic acid is shown in Equation 3.7.

Alkaline Catalyst NH,NO, P* N20 + H20

Ac~d Catalyst pH <I f

Acid Catalyst HN(NO,), - N20 + HNO,

In order to understand the reaction mechanism, two known reactions

for the formation of nitramide and dinitramidic acid are compared [16-181

The discovery of ADN led in 1994 to its synthesis by the reaction of

NH3 and N205 as shown in Equation 3.8.

4 NH, + 2 N,Oj - NH4N(N02), + 2 NH4N03

.,........ (3.8)

The reaction of N205 and NH3 and the formation of nitrarnide at

-78°C can be represented b.y Equation 3.9.

Schmitt and co-workers postulate a certain probability of forming

nitramide as an unstable intermediate at low temperatures in the course of

the formation of ADN. The reaction sequences shown in Equations 3.10 to

3.1 2 represent the formatiori of ADN.

The f~rst step of nitration of ammonia yields nitramide, whereby

nitramide reacts with a second equivalent of N205 to form the dinitramide.

A reference is hereby made to derive a possible mechanism of

dinitration of ammonium sulphamate to dinitramidic acid by comparing the

scheme postulated by Tellier-Pollon and C.Canis ''gl. The formation of

nitrarnide and dinitramidic acid shown in Equations 3.10 to 3.12 will not

give a probable mechanisrr~ of formation of dinitramidic acid in the nitration

of ammonium sulphamate. On the other hand, Tellier-Pollon has shown

that nitramide can be formed from fuming HN03 by reaction of sodium salt

of sulphamic acid [lbl (NaS03NH2). The reaction is found to follow the

scheme given in Equation 3.13.

The authors have isolated the formed nitramide at low temperatures

(-10 to -35°C) with yields ranging from 12-60%. The formation of nitramide

is further confirmed by carrying out potentiometric titration using a base.

Scheme 3.1 represented the probable mechanism of formation of

dinitramidic acid. The dinitramidic acid formation could occur via the

formation of nitramide, followed by subsequent nitration of the same to

DNA. The DNA formation could also be a concerted mechanism through

the formation of intermediate [Dl. There is no experimental data available

on the formation of either nitramide or the intermediate [Dl. Based on the

results of Tellier-Pollon, the mechanism of nitration of AS appears to

involve the formation of nitramide. Further studies are to be carried out in

order to ascertain the formation of nitramide as an intermediate in the

nitration of ammonium sulphamate.

3.3.3. Spectral and Thermal Analysis of Ammonium Dinitramide

The obtained ADN was characterized by means of UV, IR,

differential scanning calorimetry (DSC), simultaneous TG-DTA and

elemental analysis techniques.

UV spectrum of the sample was recorded in water solution.

Ammonium dinitramide shows UV maximum in water at 212 and 284nm ['I.

UV spectrum of ammonium dinitramide in water is given in Figure 3.2. The

absorbance at 284nm is characteristic of the -N(N02)- ion due to the low

energy n - n' transition and the absorption maximum at 212nm is attributed

to the high energy 0 - 0' !:ransition. The molar extinction coefficient, E284

obtained from the slope of the concentration versus absorbance plot is

5.258 x 10" mol~'cm~' and this value is comparable with the value

reported ['I by Bottaro et al., (~284 = 5.207 x 10% mol~'crn~'). The results on

the calculation of molar ext~nction co-efficient for ADN in water, methanol

and acetonitrile are given in chapter 2, section 2.6 (vide infra).

Wavelength (nm)

Figure 3.2: UV spectrum of ammonium dinitramide (ADN) in water

Infrared spectrum of ammonium dinitramide was recorded in KBr

pellet. The spectrurn shows characteristic peaks and the assignments for

the peaks are given in Table 3.1. A typical IR spectrum of ADN is given in

Figure 3.3. The peak assignments were made in comparison with the

calculated v~brational frequencies reported for ADN '20,211.

Figure 3.3: IR spectrum of ammonium dinitramide (ADN)

Table 3.1: Characteristic peaks in IR spectrum of ADN

- ~ - ~.

v N-H of NH4' ~- - ~ ~

v, NO* in phase -

- ~ - ~ p ~ ~ ~ - - ~ ~

v, NO2 out of phase -. ~~ ~.~

v,, NO2 in phase - . . . .- ~~

v,, NO* out of phase -- -~~

-p-- ~- ~.

a,,,,, NO2 in phase - ~ ---- ~. .

6,ci,, NO2 out of phase ~ ~ --

~ . ... -

ijlOCk NO2 out of phase -- ~~~~~ ~ ~~- ~ ~ ~~-p~~--~

L.S N3 - ~- ~ ~ ~ ~ . ~ . -- -~~~

~ . a 5 N3

Wave number (cm")

31 24

1343

1178

1538

1403

827

76 1

731

952

1022

Thermal study of ammonium dinitramide by DSC shows an

endotherm at 92" C corresponding to the melting followed by an exotherm

in the range of 140°C - 230" C. The exotherm is followed by an endotherm

at 250°C, due to the endothermic decomposition of in-situ formed AN. A

typical DSC of the ammonium dinitramide is given in Figure 3.4. Detailed

studies on the thermal decomposition of ADN using DSC & TG-DTA are

given in Chapter 6 (vide supra).

Figure 3.4: DSC trace of ammonium dinitramide (ADN)

The TG-DTA trace of ADN is given in Figure 3.5. The TG trace of

ADN showed a s~ngle stage decomposition pattern with 100% mass loss at

220°C. DTA trace of ADN showed a melting endotherm at 92°C followed by

exothermic decomposition n the temperature range of 150 - 210°C with a

300

3 - 2-

- 1- -

O- q -,: -2 - -3 -

peak maximum at 172°C. The DTA trace also shows an endotherm at

214"C, which is due to the endothermic decomposition of ammonium

nitrate in-situ formed during the decomposition of ADN.

1827OC

9066°C: 1 W.2 Jlg -- Y I

158.30"C 1896.2 Jig

92 "C i I I I I I

50 100 150 200 250

Figure 3.5: TG-DTA trace of ammonium dinitramide (ADN)

The elemental analysis results of ADN agree with the calculated

values. The calculated and found values are given in Table 3.2.

Table 3.2: CHN analysis results of ADN

3.3.4. Effect of Variation of Acid Ratio on the Yield of Dinitramidic Acid

Calculated -~

C 1 Nil

H 3.23%

N 45.16%

When experiments were carried using the stoichiometric ratio of 2,

ie., with sulphuric acid (SA, H2S04) to nitric acid (NA, HN03) in the mole

Found

0.27%

2.87%

44.14%

ratio of 2:1, maximum yield of 45% was obtained in 20min. The acid

mixture, however, is very viscous at - 40 "C hence fast addition of the

reactant (AS) at a controlled rate becomes very difficult.

Based on the Zamen's Raman spectral measurements L231, the mole

% of nitronium Ion NO2' in the mixed acids containing 50 mole% H2S04

and 50 mole% HN03, is 14. Therefore, the mixture containing 0.1M each

of H2S04 and HNO3, the niolar concentration of nitronium ion would be

0.028M. When the mixture contains 0.2M each of HzS04 and HN03 the

molar concentration of NO2' would be 0.056M. It is seen that as the molar

concentration of the acids increased further, the NO; concentration also

would increase correspondingly. When equimolar mixture of acids was

taken in quantities as shown in Table 3.3, the yield of dinitramidic acid

increases from 6% to 37%. The yield of DNA for the mole ratio 1: l is 6%

and the mole ratio 3.5:3.5 is 37%. As the total acid concentration increased

further the reaction mixture becomes very viscous and hence further

increase of acid concentration was not attempted.

Table 3.3: Yield of din~tramidic acid after reaction time of 20 min

SI.No 1 Mole ratio I DNA yield * (%) SA:NA

The yield of dinitramidic acid during nitration depends on important

parameters like viscosity of the medium, temperature, agitation speed and

rate of addition of the reactants. To reduce the viscosity of the medium at

temperatures below -40°C, Inore nitric acid was taken in the system, which

will act as a solvent. The decomposition of the dinitramidic acid in acid

6

26

37

1 i i 1: l I

based on UV spectroscopy

2

3

2:2

3.5:3.5

medium also becomes a critical factor in determining the yield of the

product.

These factors can be overcome by increasing the rate of agitation of

the reaction m~xture, cooling the reactor at much faster rate and also by

making the system less viscous so that the transfer of nitronium ion to the

substrate takes place efft:ctively. In order to make the reactant well

dispersed in the medium, experiments were carried out using SA:NA in the

mole ratio of 2:6. In a typical experiment the yield of dinitramidic acid was

measured at d~fferent times and the results are shown in Table 3.4.

Table 3.4: Yield of dinitramidic acid at different times

I Time (min) I % Yield of DNA* (for ratio of SAINA 2:6)

As it is seen from Table 3.4, the yield of dinitramidic acid increases

from 17O/0 to 35%.

35

3.3.5. The Effect of Nitric Acid on the Dinitramidic Acid Yield

35

The effect of nitric acid on the dinitramidic acid yield was studied by

varying the nitric acid concentration from 2 moles to 12 moles keeping the

concentration of sulphuric acid at 2 moles. The results are given in Table

3.5 for the reaction carried out using 1 mole of AS at -45°C for a reaction

time of 30min.

* based on UV spectroscopy

Table 3.5: Yield of DNA for different ratios of SAINA

From Table 3.5 it is seen that as the mole percent of nitric acid in the

reaction mixture increases, the yield of dinitramidic acid shows an increase

from 26% to 35%. This is because the excess nitric acid present in the

system acts as a solvent and makes the system less viscous so that

effective nitration of the substituted arnine takes place thereby improving

the yield. The yield however has not gone beyond 35% even when the

nitric acid was increased above 12%.

Sulphuric acid /Nitric acid mole ratio

~~

2:2

2:6

2:lO

2:12

3.3.6. The Effect of Sulphuric Acid Ratio on the Rate of Formation of Dinitramidic Acid

% yield of DNA at 30min

26

30

3 1

35

In order to further study the effect of variation of sulphuric acid ratio

on the rate of formation of dinitrarnidic acid, experiments were carried out

for fixed quantities of AS and nitric acid, by varying the sulphuric acid. The

rate of formation of dinitrarnidic acid was monitored by taking in-process

samples of the reaction mixture and then analyzing it for the amount of

dinitramidic acid formed. The obtained rates for different ratios of sulphuric

acid and nitric acid are given in Table 3.6.

Table 3.6: Rates of formation of DNA for different ratios of sulphuric acid

Ratio of sulphuric acid I nitric acid

0.0183

It is seen from Table :3.6, that for the system with no sulphuric acid

the rate is 0.018:3, while other systems with varying ratios of sulphuric acid,

show a phenomenal increase in the rate indicating the catalytic effect of

sulphuric acid. Maximum rate was obtained for the ratio of 4:8. However,

on increasing the sulphuric acid concentration further to 8 moles, the rate

has drastically come down. The reason for this is the heterogeneity of the

mixture occurring due to solidification of the sulphuric acid rich reaction

mixture at low temperature. This prevents the effective attack of nitronium

ion to the substrate in the nitrating mixture.

3.3.7. The Effect o f Water on the Yield of Ammonium Dinitramide

Having studied the effect of sulphuric acid and nitric acid on the

formation of dinitramidic acid, and to improve the yield of dinitramidic acid,

experiments were carried out by adding different quantities of water to the

nitrating mixture. The water concentration was varied from 1 to 2.3M, and

the formation of ammonuum dinitramide was studied. In a typical

experiment for lmole of AS and SNNA ratio 2:12, the increase in water

concentration upto 1.7M showed an increase in the yield of ammonium

dinitramide from 30% to 5O0/0. This is because the added water molecule

facilitates formation of dinitramidic acid from the nitrated intermediate as

shown in Equations 3.14 to 3.18.

I NO, [ Bl [ Cl .......... (3.15)

[B] + NO,' -.- (N02)2NS03NH4 [Dl .......... (3.16)

[ C] + NO,' .- HN(N02), ......... .(3.17)

When water content is very low, the conversion of the intermediates

B and D to dinitrarnidic acid is poor, thereby reducing the yield. When the

water content is about 1.33M for 1M of AS, Equation 3.14 is a predominant

step thereby resulting n the favourable NH~NOZ. AS the water

concentration is increased beyond that, the mono nitro compound itself

undergoes hydrolysis leading to the formation of hydrolyzed products

(equation 3.19), which are unstable under these reaction conditions.

Hence large concentration of water is detrimental to get maximum yield.

Table 3.7 glves the results for the reaction carried out for 1M of AS and

with 2 moles of sulphuric acid and 12 moles of nitric acid.

excess H20 NH,N02 N 2 0 + H 2 0

Table 3.7: Effect of water on the yield of ADM

. .:

I . . .. . .

Yield of ADN * (%) ~-. .- .-

Water (mole) I

I ; 42.5

52.0

2 3 I 46.0

* By UV spectroscopy

With the add~tion of 1.7M of water, the yield of ammonium

dinitramide was ~ncreased to 52%. The reproducibility of the results at the

same level was tested and the results obtained show good reproducibility.

In all the experiments the obtained yield was above 50% for a water

concentration of 1 7M.

3.3.8. The Effect of Variation of Temperature

The effect of variatior of temperature was studied for the reaction

carried out using a SAINA mole ratio of 2:6 for l M of AS at -65", -55", and

-45°C. The reaction rate was found to be very slow at -65°C and slowly

increase at -55°C and a maxlmum rate is obtained at -45°C.

A$ seen from Figure :3.6, at above -45"C, run-away reaction takes

place because of the high exothermic nature of the reaction, the instability

of the intermediates and also due to the instability of the formed

dinitramidic acid at this temperature. At temperatures below -65"C, the

reaction becomes too slow due to high viscosity of the medium and also it

does not provide the required activation for the formation of mono nitro

intermediate leading to very poor yield.

Time

Figure 3.6: Rate of reaction at different temperatures

3.3.9. The Effect of Using Solvents as Nitrating Medium

In order to promote better stirring and effective heat transfer, inert

solvent namely CH2CI2 was used as a nitrating medium. In all the

experiments, the obtained yield of dinitramidic acid was very low. The yield

was less than 5%. This is because the reaction mixture was totally

insoluble in the solvent, it becomes heterogeneous and the solvent layer

separates from the reaction mixture thereby making the system highly in-

homogeneous and effective reaction does not takes place.

3.3.10. Summary of Yield of ADN for Different Ratios of SAlNA

Different ratios of SA/NA were studied in the nitration of ammonium

sulpharnate at a temperature of -45°C. The results obtained from the study

are given as a bar chart in Figure 3.7.

SPJNA ratio

Figure 3.7: Yield of ammonium dinitrarnide for different ratios of SAINA

As it is seen from Figure 3.6, the yield of ADN increases from 16%

to 35% as the concentration of nitric acid increases. Further improvement

in yield was achieved by addition of water. Increasing the sulphuric acid

concentration to 4 moles, the yield of ADN decreases as in the case of

ratios 4:2 to 4:12

PART-II

3.4. Synthesis of Potassium Dinitrarnide (KDN)

Potassium dinitramide KN(NO& has attracted a wide interest as a

promising new class of energetic oxidizer, which finds applications as an

energetic phase stabilizer in ammonium nitrate (AN) based propellants

[24,251, a dinitramide transfer reagent '261. It also finds applications in various

pyrotechnic formulations. The synthesis of potassium dinitramide was

carried out by two methods. In the first method, potassium sulphamate

KS03NH2 was nitrated using mixed acids under the experimental conditions

given in chapter 3, section 3.2.2. The second method of synthesis involves

the double decomposition of ADN with potassium hydroxide in a solvent.

3.5. Experimental

3.5.1. Materials

Potassium sulphamate was prepared by neutralization of sulphamic

acid NH2S03H with KOH. Sulphamic acid (48.59, 0.5mol) was dissolved in

50ml of water. KOH (289, 0.5mol) was dissolved in 50ml of water and both

the solutions were mixed at 0°C. The resultant solution (pH 7+1) was

precipitated in methanol. The precipitated potassium sulphamate was

filtered, washed with methanol and dried in an oven at 70°C for 2 hrs.

KOH (NICE chemicals, Cochin) was used as received. Solvents

viz., isopropanol. acetone, methanol were distilled and dried over molecular

sieves prior to use. ADN with purity > 98% was synthesized as explained

in section 3 2.2. Con. H2S04 98% (Qualigens, Mumbai) was used as

received. Fuming HN03 of assay > 98% was distilled in the laboratory from

a mixture of 1 : I (by welght:) of fuming HN03(92% ) with con. H2S04. The

fraction between 83 - 85°C was used.

3.5.2. Method 1

In a typical experiment potassium sulphamate (6.759, 0.05mol) was

added in small portions to a mixture containing HN03 (assay-98%, 18.9g,

0.3mol) and con.H2S04 (9 89, 0.lmol) at a temperature of -35" C to

-45" C in a 500ml jacketed reactor with stirring. The rate of addition was

controlled in such a way that the temperature of the mixture does not

exceed -35" C Formation of white precipitate was o b s e ~ e d during the

course of the reaction due to the precipitation of potassium sulphamate and

the viscosity of the mixture increases as the reaction progresses. Stirring

of the contents was continued for a pre determined time and then the

reaction mixture was diluted by pouring into about 1009 of crushed ice.

The diluted acid solution was neutralized immediately by adding cold KOH

solution while maintaining the temperature below 0°C. The pH of the

solution was checked during the course of the neutralisation and it was

continued till the solution becomes alkaline (pH - 7.5 to 8). The reaction

sequences are given in Equations 3.20 and 3.21.

KS03NH2 + 2 NO,' + 2 HSO; + H20 - HN(N02), + 2 H,SO, + KHSO,

HN(N02), + KOH ---* KN(NO,), + H20 .......... (3.21)

The neutralized solution was diluted to known concentration and

analyzed by UV spectroscopy. The yield of the ammonium dinitramide was

calculated by measuring the absorbance at 284nm and then calculating the

concentration using the calc:ulated molar extinction co-efficient ( ~ ~ ~ 4 = 5.379

x l o 3 L mol-'cm'). The calculation of molar extinction co-efficient for KDN

was given in chapter 2 section 2.6.6 (vide infra)

3.5.3. Method 2

This involves a sirnple procedure. Potassium hydroxide (6.69,

0.lmol) was dissolved in 100ml of dry CH30H. Ammonium dinitramide

(12.49, b . lmo~ ) was dissolved in another 100ml of CH30H. The two

solutions were combined and kept in a freezer for 2-3 hrs, the resulting

crystalline solid of KDN was collected by filtration and dried under vacuum

for 1-2 hrs. The yield is 90-92%.

3.5.4. Separation of Potassium Dinitramide

The neutralized solution (from method 1) was evaporated under

vacuum to remove water. The evaporated solid was further dried in

vacuum to a dry powder. The dried solids were then extracted with

acetone in portions, concentrated under vacuum and precipitated in

isopropanol to get a crystalline white solid. This was purified further by

recrystallization from methanol to get pure potassium dinitramide. The yield

is 75-80%.

3.6. Results and Discussion

3.6.1. Characterization of Potassium Dinitramide

KDN was characterized by FTIR, simultaneous thermogravimetry

differential thermal analysis (TG-DTA) and elemental analysis. FT-IR

spectrum of potassium dinitramide IS given in Figure 3.8. The characteristic

IR peaks for KDN are given in Table 3.8.

Wave Number (cm')

Figure 3.8: FT-IR Spectrum of potassium dinitrarnide

The peak assignments in IR spectrum for KDN were made based on

the calculated vibrational frequencies for KDN L20.211

Table 3.8: Characteristic peaks in IR spectrum of KDN

- . ~ - - - ~ - ~

v,, NO2 in phase -~ ~ ~~~

V, NO2 /n phase -~ .

Wave number (cm")

1530

1384

V, NO2 out of phase ! 1196 . . ~

6,,,,, NO2 in phase : ~

827 +-- 6,,,,s NO2 out of phase I 761

.. ~

firock NO2 out of phase f 731 ~ ~ I

vas N3 1023 ~~ ~- ~~ ~ ~ t-

vs N3 951

The TG-DTA trace of KDN [271 is given in Figure 3.9. TG trace of

KDN showed a mass loss of 57% at 250°C. The DTA trace of KDN

showed a melting endotherm at 128°C followed by an exothermic

decomposition in the temperature range of 170-260°C with a peak

maximum at 230°C. The DTA trace also showed an endotherm at 308"C,

which is contributed to the melting endotherm of in-situ formed potassium

nitrate in the decomposition of KDN.

Figure 3.9: TG-DTA trace of potassium dinitramide (KDN)

The percentage of nitrogen value obtained is closer to the calculated

value. (Found: 28.82%. Calc:ulated: 28.96%).

3.7. Synthesis of Guanylurea Dinitramide (GUDN)

Guanylurea dinitramide is a new energetic material with low

sensitivity. Unlike other dinitramide salts it is neither soluble in cold water

nor hygroscopic [28,'91. This prompted us to study its synthesis and

characterization. The synthesis of guanylurea dinitramide involves two

steps. The first step is the formation of guanylurea sulphate. The treatment

of guanylurea sulphate in water with a water solution of ADN results in the

formation of guanylurea dinitramide as fine powder.

3.8. Experimental

3.8.1. Materials

Dicyandiamide (CDH, Mumbai) & Con.H2S04 (Qualigens, Mumbai)

were used as received. lsopropanol (Qualigens, Mumbai) was distilled and

dried over molecular sieves. Ammonium dinitramide (ADN) was prepared

by the procedure described in chapter 3, section 3.2.2. The purity of ADN

used is >98%.

3.8.2. Preparation of Guanylurea Sulphate

Dicyandiamide (16.89, 0.2mol) was treated with aqueous sulphuric

acid (9.89, 0.1 mol in 50ml of water) and kept over a hot water bath for 3-4

hrs. The resulting solut~on was kept in an ice bath for 1 hr, the formed

white crystals were filtered off. The rest of the filtrate was concentrated

over a hot water bath and then cooled in ice. The resulting crystals were

filtered, combined, washed with isopropanol and then dried in an hot air

oven. The yield is 88-90%. The reaction sequence is given in Equation

3.22.

3.8.3. Preparation of Guanylurea Dinitramide

Guanylurea sulphate (30.29, 0.lmol) was dissolved in 50ml of water

with slight warming and stirring. When the solution is clear it was taken out

and cooled to room temperature. ADN (24.89, 0.2mol) dissolved in 10ml of

water was added to the above solution in portions. A fine white crystals

were formed. This was filtered, washed several times with cold water and

finally dried under vacuum for an hour. The yield is of the same is 90-95%.

The reaction is glven in Equation 3.23.

3.9. Results and Discussion

3.9.1. Characterization of GUDN

The purity of GUDN was determined from elemental analysis, the

values are given in Table 3.9. IR spectrum of GUDN is given in Figure 3.10.

The IR peak assignments for GUDN are given in Table 3.10.

Table 3.9: CHN analysis of guanylurea dinitramide

C

H

N

Calculated ~

11.48%

3.35%

46.89%

Found

11.70%

2.80%

46.90%

2WO 1500

Wave Number (cm")

Figure 3.10: FT-IR Spectrum of guanylurea dinitramide (GUDN)

Table 3.10: Characteristic peaks in IR spectrum of GUDN

-- ~ ~~~

v,, NO2 in phase

vs NO2 in phase ~ -

v, NO2 out of phase

6sciss NO2 in phase >

6,,is, NO2 out of phase ~

Srock NO2 out of phase -- ~

vas N3 - ~ .---

vs N3 .. ~ ~ - - ~ ~~~ ~

N-H stretching .

C=O stretching - --

=C=NH stretching

Wave number (cm")

1520

1331

1171

81 6

744

703

1013

- 914

3439,3325.3239

1635

1688

3.10. Preparation of Tetramine Cu(ll) Dinitramide

A complex salt of dinitramide was prepared employing ADN, copper

sulphate and ammonia solution ["I. The isolation of [ C U ( N H ~ ) ~ ] [ N ( N ~ ~ ) ~ ] ~

was achieved by mixing of ADN, CuS04.5H20 and liquor ammonia in

water.

3.1 1. Experimental

3.1 1 . I . Materials

CuS04.5H20 (Purex Laboratories, Bangalore) and liquor ammonia

(Qualigens, Mumbai) were used as received. ADN was synthesized by the

procedure described in chapter 3, section 3.2.2. The purity of ADN used in

the study is >98%.

3.11.2. Synthesis of Tetramine Cu(ll) Dinitramide

A saturated solution of CuS04.5H20 (3.7g, 0.015mol) in aqueous

NH3 was reacted with a saturated solution of ADN (3.79, 0.03mol) in H20

and cooled to 0°C producing violet crystals of tetramine Cu(ll) dinitramide

[CU(NH~)~] [N(NO~)Z]~. The formed crystals were filtered and were dried in a

desiccator. The y~eld is 9045%.

3.12. Results and Discussion

IR and elemental analysis of tetramine Cu(ll) dinitramide was

performed and the results are given Figure 3.11 and Table 3.11

respectively.

Table 3.11: CHN analysis of tetramine Cu(ll) dinitramide

Calculated

t ~

Nil

H 3.49%

N 40.76%

Found

Nil

2.9 %

40.3%

28

26

24

P

18

14

10

6

40M) 2WO 1500

Wave Number (cm")

Figure 3.1 1: FT-IR spectrum of tetramine Cu(ll) dinitramide

FT-IR spectrum of tetramine Cu(ll) dinitramide shows characteristic

peaks and is given in Table 3.12.

Table 3.12: Characteristic peaks in IR spectrum of tetramine Cu(ll) dinitramide

I Wave number fcrn-ll

v, NO2 in phase ~

6,,i,, NO2 in phase

v, NO2 out of phase -- 6,,,,, NP2 out of phase

~

6,,,1, NO2 out of phase 733

3.13. Emulsion Crystallization of Ammonium Dinitramide (ADN)

Solid propellants often consist of heavily laden polymer systems. In

these, the particulate components that can comprise upto 90% by weight of

the total mass are especially important. The formation of these spherical

particles by recrystallization or other means is a production step, which

follows after the synthesis of the compound to give a finished solid

propellant or explosive charge. Emulsion crystallization is particularly suited

to recrystallization of fusible propellant and explosive materials[301.

Ammonium dinitramide crystallizes naturally in the form of needles

or plates, which are not readily amenable to subsequent processing. For

use in propellant it is necessary to use solid particulate ADN of controlled

size to obtain predictable results. Particles in the range of about 50 to 500

pm are considered useful [3'-331. Efforts to control crystallization or to

physically process solid ADN to obtain a selected particle size have been

unsuccessful. Hence a suitable method for preparing spherical grains of

ADN was worked out. The process by which spherical ADN can be

produced is explained below.

3.14. Experimental

3.14.1. Materials

Ammonium dinitrarr~ide with purity >98% and a mean particle size of

40-501rrn was used. The synthetic procedure was described in chapter 3.

section 3.2.2.

Paraffin oil (XCELTHERM 600, Radco Industries lnc., USA) was

used as received. The specific gravity is 0.82 to 0.88. Methylene chloride

(AR) (SRL laboratories, Mumbai) was used as received.

Fumed silica (Cab-0-31) with a mean particle size of 50nm was

used as a protective colloid.

3.15. Emulsion Crystallization Process

The emulsion crystallization process involves the melting of ADN in

paraffin oil maintained above the melting temperature of ADN.

Subsequently, the molten ADN in the paraffin oil is stirred to get uniform

spherical grains of ADN. Finally cooling the paraffin oil to room

temperature and isolation of spherical grains by washing with a solvent.

The process diagram for the emulsion crystallization process is

given in Figure 3.12 .In a typical experiment 209 of dried ADN was slowly

introduced into hot paraffin oil maintained at 92-93°C in a jacketed reactor

(JR), along with 0.1% of fumed silica (Cab-0-Sil) as a protective colloid to

prevent particle-particle adhesion. Temperature of the paraffin oil was

controlled using the heating and cooling exchanger (HCE). The hot paraffin

oil along with the molten ADN was mechanically stirred using a propeller

stirrer (M) at 400-500 rpm.

Ethylene glycol -i- H20

Paraffin Oil

tieatlng and i--~---J [JRI Cooling Exchanger

Figure 3.12: Schematic representation of emulsion crystallization process

Afler the formation of visible spherical molten ADN grains, the

paraffin oil was slowly cooled to room temperature by circulating water

through the jacketed reactor. The molten ADN spherical grains will

crystallize as the temperature is lowered. The spherical ADN particles

were then filtered off to remove the paraffin oil and washed twice or thrice

with methylene chloride to remove adhered paraffin oil on the surface of

ADN. The methylene chloride washed ADN was dried further under

vacuum and stored in a desiccator.

3.16. Results and Discussion

Table 3.1 3 shows the results obtained in the emulsion crystallization

process of ADN with different quantities of starting material.

Table 3.13: Batch-wise results on the emulsion crystallization of ADN

' with a battle

It is seen from Table 5.2. for batches EC-I to Ill, the particle size

obtained was above 300prn. Further improvement to control the particle

size was made by using a baffle in the reactor which promotes uniform

stirring and avoids the turbulence of paraffin oil. The results shown in

Table 5.2 for EC-IV and EC-V are obtained using a baffle. The results

show that very good particle size distribution is achieved by using a baffle.

I

Batch ADN (g) Cab-0-Sil 4 (g)

. -- . -

EC-I a , 0 027

Paraffin oil (ml)

200

200

400

500

600

EC-II 15

EC-Ill , 25 I

EC-IV a 25

EC-V a 1 30 I

Q -.

0 2

0.050

0 056

0 093

Stirring time (min)

10

5

5

15

10

Particle size (pm)

>300

2500

>500

100-400

200-600

3.16.1. Particle Size Analysis

The particle size of emulsion crystallized ADN was analysed by

optical microscopy. Figures 3.13 and 3.14 show the optical microscopic

images of fine and coarse ADN respectively. It is seen that uneven needle

shaped crystals are observed for fine ADN, while the emulsion crystallized

coarse ADN showed smooth spherical surfaces.

Figure 3.13: Optical microscopic image of as-synthesized ADN

Figure 3.14: Optical microscopic image of emulsion crystallized ADN

117

3.16.2. Analysis of Emulsion Crystallized ADN

The influence of emulsion crystallization on ADN is studied by IR

and differential scanning calorimetry (DSC). The IR spectra and DSC of as-

synthesised ADN was compared with that of the emulsion crystallized ADN.

Figure 3.15 shows the overlay of IR spectrum of as-synthesised ADN and

emulsion crystallized ADN. It is seen from Figure 3.15, that all the

characteristic peaks for ADN are present in both the spectra. The ADN

particles formed by the emulsion crystallization process show no significant

changes.

I I I I I 4WO 3WO

I 2WO 1m 10M 650

Wavenurrber (uri')

Figure 3.15: Overlay of IR spectra of ADN (as-synthesised) and ADN (Emul. Cryst)

Figure 3.16 shows the DSC overlay of ADN (as-synthesised) and

ADN (emul.cryst). Comparison of the DSC curves shows no difference in

the melting and decomposition pattern. The emulsion crystallization

process does not seem to affect the properties of ADN.

Tenperature ec)

Figure 3.16: DSC overlay of ADN (as-synthesised) and ADN (EmuLCryst.)

3.17. Conclusions

The nitration of ammonium sulphamate using mixed acids was

studied. In the nitration of ammonium sulphamate, the formation of

dinitramidic acid depends on the mole ratio of the sulphuric acidlnitric acid

taken and the time of the reaction. Maximum yield of dinitramidic acid 45%

was obtained in 20mts when the ratio of SAINA is 2:l. The addition of

sulphuric acid to the nitric acid catalyses the reaction. The effect of

variation of nitric acid and sulphuric acid on the formation of dinitramidic

acid was studied. Maximum rate was obtained for SNNA ratio 4:8. The

addition of water to the nitrating medium helps in improving the yield of

ammonium dinitramide upto 50%. None of the solvents were proved to be

an efficient nitration medium. The study on the effect of temperature on the

formation of dinitramidic acid shows that for optimum reaction, a

temperature of -45°C is necessary, above which the reaction is highly

exothermic and below which the reaction is too slow. The study given in

this chapter helped optimizing the reaction conditions for getting a higher

yield of ammonium dinitrarnide.

Dinitramide salts viz., potassium dinitramide, guanylurea dinitramide

ar~d tetramine Cu(1l) dinitramide were synthesized and characterized by IR

and elemental analysis. The synthetic routes either involve a nitration

reaction or a simple double decomposition. The synthesized dinitramide

salts were thermally characterized by thermo analytical methods.

Elaborate thermal studies are given in chapter 6 (vide supra). The above

salts will find applications in inorganic and organic synthesis. A process for

the formation of spherical grains of ADN from fine ADN was achieved by an

emulsion crystallization process. The described process enables

production of spher~cal grains of ADN with a mean particle size of 100-

500pm. The analytical results by IR and DSC show no significant

difference between the as-synthesized ADN and the emulsion crystallized

ADN.

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