Effects of temperature on nitration of sulfamates

Yuji Sugie • Atsumi Miyake

Received: 30 August 2013 / Accepted: 3 February 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Ammonium dinitramide (ADN) has attracted

great interest as a potential oxidizer for next generation

rocket propellants. It is a halogen-free alternative to

ammonium perchlorate, which is currently in wide used as

a solid propellant oxidizer. However, in ADN synthesis,

N-nitration is necessary to form the N-(NO2)2 group. Using

a reaction calorimeter, the thermal behavior of nitration of

sulfamates (K, Na, and NH4) using a mixture of acids

(HNO3/H2SO4 and HNO3/AcOH) as the nitration agent

was examined. The heat of decomposition of potassium

sulfamate at -10 �C was greater than that at 20 �C. The

heat of decomposition decreased in the following order: K

salt[Na salt[NH4 salt in HNO3/H2SO4. The dipole

moments of the sulfamates were calculated, and the results

revealed that the electronic states of nitrogen were differ-

ent. Thus, the dipole moments of sulfamates affect the

decomposition heat of sulfamates. The heat of decompo-

sition in HNO3/AcOH was larger than that in HNO3/

H2SO4.

Keywords Sulfamate � Nitration � Reaction calorimetry �Electronic state

Introduction

Ammonium perchlorate is widely used in solid propellants,

but releases HCl during burning. For this reason, much

research has been carried out to find suitable chlorine-free

oxidizers [1–4]. Ammonium dinitramide (ADN) is a

promising chlorine-free alternative oxidizer for next gen-

eration rocket propellants. Ammonium dinitramide is

obtained from organic or inorganic compounds via a few

steps. The number of steps in ADN synthesis from organic

compounds usually is more than that from inorganic

compounds. Because ADN is an inorganic compound,

transformation into an inorganic compound is required if

an organic compound is used as the starting material.

However, as ADN contains an N-(NO2)2 group, N-nitration

is needed for ADN synthesis. Nitrating agents used for

ADN synthesis include N2O5, NO2BF4, and mixed acid

solutions [5–9]. N-nitration is a key reaction for ADN

synthesis and affects the total yield. N-nitration occurs on

the nitrogen that includes an amino or isocyanate group,

usually at a reaction temperature below 0 �C. Strong

nitrating agents are often used, and the nitration proceeds

one step at a time. The nitration yield varies widely

depending on the raw material, reaction temperature, and

nitrating agent. In the synthesis of potassium dinitramide

(KDN) (Scheme 1) [5], the nitration of potassium sulfa-

mate occurs in the presence of HNO3/H2SO4.

KDN is an important precursor of ADN, and ADN can

be produced easily from KDN. The nitration of potassium

sulfamate must be conducted at temperatures lower than

-40 �C. Dinitramidic acid (HDN) is the intermediate

product generated, but it is very unstable even at -30 �C.

The HDN easily decomposes to HNO3 and N2O at higher

temperatures [10–13]. Under less than ideal nitration con-

ditions, exothermal decomposition and gas formation

Y. Sugie (&)

Japan Carlit, 1-17-10 Kyobashi Chuo-ku, Tokyo 104-0031,

Japan

e-mail: sugie-yuji-jp@ynu.jp; sugie@carlit.co.jp

A. Miyake

Yokohama National University, 79-7 Tokiwadai Hodogaya-ku,

Yokohama, Kanagawa 240-8501, Japan

e-mail: atsumi@ynu.ac.jp

123

J Therm Anal Calorim

DOI 10.1007/s10973-014-3688-4

occur. Therefore, control over the reaction temperature and

reaction procedure must be maintained to perform the

nitration safely. The present study focuses on the nitration

of sulfamates for safe ADN synthesis. The reaction was

observed under undesirable nitration conditions, and the

effects of the temperature and the nitration agent on sul-

famate nitration were investigated by measuring the heat of

decomposition using reaction calorimetry. Dipole moments

of the sulfamates also were calculated.

Experimental

Materials

All sulfamates were prepared by neutralization of sulfamic

acid as shown in Scheme 2 [5].

Sulfamic acid was dissolved in water at room temperature

and the pH was adjusted to 7 using 10 % aq. KOH. The

mixture was concentrated using an evaporator about three-

quarters and 2-propanol was used to evaporate the water. The

precipitate was isolated by filtration and washed with etha-

nol. The potassium sulfamate obtained was recrystallized

from a mixed solvent of 2-propanol and water. Potassium

sulfamate was obtained as a colorless solid. The other sul-

famates were obtained as colorless solids using the same

method. The sulfamates were characterized by elemental

analysis (Vario, EL CHNOS Elemental Analyzer) and

Fourier-transform infrared spectrometry (Jasco, FT/IR-420).

Observations during undesirable nitration

A mixture of fuming HNO3 (45 mL) and conc. H2SO4

(16 mL) was stirred and cooled to -40 �C in a glass

beaker. Then, potassium sulfamate (17 g) was added and

one of the following two conditions was maintained: (1)

cooling was continued but the mixture was not stirred, and

(2) cooling was stopped in about 40 min after addition but

the mixture was stirred. All experiments were recorded

using a CCD camera. The reaction temperature was mon-

itored using a type K thermocouple coated with

polytetrafluoroethylene.

Measurement of decomposition heat

The heat of decomposition was measured using a reaction

calorimeter (OmniCal, SuperCRC) in isothermal mode, as

shown Fig. 1.

The temperature was adjusted to 20, 0, and -10 �C. In

the glass vial, sulfamate (12 mg) was suspended in conc.

H2SO4 (0.2 mL) or acetic acid (0.22 mL) and cooled to the

set temperature. Then, fuming HNO3 (0.5 mL) was drop-

ped into the solution from a plastic syringe set above the

sample vial. An empty vial was used as a reference. At

least three measurements were obtained at each tempera-

ture for each sample. Before measuring the heat of

decomposition, the heat of mixing of conc. H2SO4 and

fuming HNO3 was measured at each temperature in various

mass. Then, the calibration curve was drawn using the

obtained data of heat of mixing and mass data. Using the

calibration curve, the heat of mixing of mixed acid was

obtained at each experiment. The average of values

obtained by subtracting the heat of mixing from the gross

calorific value observed was used in the data analysis.

However, the heat of decomposition included the heat of

nitration reaction in the present study.

H2NS

OK

O

O

NO2+

HN(NO2)2KOH KN(NO2)2 NH4N(NO2)2

temperature increase N2O+ HNO3

Scheme 1 Synthesis of ADN from potassium sulfamate

HOS

NH2

O

O

ROHRO

SNH2

O

O

R=Na, K, NH4

H2O

Scheme 2 Synthesis of sulfamates from sulfuric acid

Fuming HNO3 (0.5 mL)

Stainless needle for vent

Sulfamate (0.12 mg) in conc. H2SO4 (0.2 mL) or acetic acid (0.22 mL)

Stir bar

Fig. 1 Schematic diagram of a reaction calorimeter SuperCRC

Y. Sugie, A. Miyake

123

Results and discussion

Nitration of potassium sulfamate

Figure 2 shows the effects of either without stirring or

stopping cooling in nitration of potassium sulfamate.

Before addition of potassium sulfamate, the mixed acid

solution was transparent. Under both conditions, a brown

gas formed upon the addition of potassium sulfamate or

stopping cooling. Figure 3 shows the temperature curves

obtained under both conditions.

Without stirring, a rapid (within 1–2 min) temperature

rise of about 50 �C was observed. A temperature rise of

about 10 �C occurred by addition of potassium sulfamate

(stopping cooling). When cooling was stopped in about

40 min, a temperature rise of about 80 �C occurred.

SuperCRC experiments

The thermal behavior of decomposition of potassium sul-

famate in HNO3/H2SO4 obtained from the reaction calo-

rimeter was shown in Fig. 4.

The thermal behavior was observed with reproducibility.

The heat of decomposition of potassium sulfamate at dif-

ferent temperatures in HNO3/H2SO4 is shown in Table 1.

Fig. 2 Photographs of

potassium sulfamate nitration,

a before addition of potassium

sulfamate, b after addition of

potassium sulfamate without

stirring, and c after addition of

potassium sulfamate without

cooling

–50

–30

–10

10

30

50

0 20 40 60

Tem

pera

ture

/°C

Time/min

Fig. 3 Temperature curves from potassium sulfamate nitration

without stirring (solid line) and stopping cooling (dashed line)

00

100

200

300

–10 °C

0 °C

20 °C

10 20

Time/min

Hea

t flo

w/m

W

30

Fig. 4 Thermal behavior of decomposition of potassium sulfamate in

HNO3/H2SO4

Effects of temperature on nitration of sulfamates

123

The heat of decomposition at -10 �C was greater than

that at 20 �C. Sulfamic acid decomposes to H2SO4, H2O,

and N2O upon addition of HNO3 [14]. Therefore, under the

same nitration conditions, potassium sulfamate is proposed

to decompose as shown in Scheme 3.

Potassium sulfamate can undergo direct decomposition

to KHSO4, H2O, and N2O in the presence of HNO3

(pathway 1). However, it also can decompose to N2O and

HNO3 after forming HDN (pathway 2). These two

decomposition pathways occur simultaneously. However,

the temperature affects the contribution of each pathway to

the decomposition process. At low temperatures, decom-

position after nitration (pathway 2) is likely to occur, and at

high temperature, decomposition by HNO3 is likely to

occur (pathway 1). The heat of decomposition is large at

low temperatures because unstable HDN decomposes.

Therefore, the heat of decomposition at -10 �C is larger

than that at 20 �C. This means lower temperature is ther-

mally hazardous in undesired condition. The heat of

decomposition of other sulfamates was also measured in

HNO3/H2SO4 using the same technique (Table 2).

The heat of decomposition of sulfamic acid at 20 �C was

greater than that at 0 �C. Because sulfamic acid is less

reactive toward nitration at 0 �C, nitration and decompo-

sition are difficult to progress. The heat of decomposition

of the other sulfamates at 0 �C was greater than those at

20 �C for the same reason in the case of potassium sulfa-

mate. The electronic state of nitrogen affects the reactivity

for nitration because NO2? attacks the sulfamate nitrogen.

Sulfamates have a tautomer that forms zwitterions (Fig. 5)

[15].

The electric state of nitrogen is different in each sulfa-

mate because the charge bias is different. The dipole

moments of the sulfamates were calculated using MOPAC

7.0 with the AM1 Hamiltonian. The results are shown in

Fig. 6 and Table 3.

The nitrogen in sulfamic acid has a slight positive

charge, which means that NO2? is less likely to attack. For

this reason, sulfamic acid has low reactivity toward nitra-

tion, and decomposition by HNO3 occurs instead. Hence,

the heat of decomposition of sulfamic acid at 0 �C is low.

For other sulfamates, the nitrogen has a slight negative

charge. Therefore, these sulfamates are more reactive

toward nitration. Because the dipole moment of sulfamate

is related to reactivity toward nitration, the dipole moment

of sulfamates affects the heat of decomposition of sulfamic

acid. The dipole moments of sulfamates and the heats of

decomposition decrease in the order: K[Na[NH4.

During the nitration process, NO2? is generated from

HNO3 upon the addition of acid [16]. Therefore, the ease of

progression of nitration is affected by the strength of the

acid. The heats of decomposition in HNO3/AcOH and in

HNO3/H2SO4 were measured at 20 and 0 �C (Table 4).

Table 1 Heat of decomposition of potassium sulfamate in HNO3/

H2SO4

Temperature/8C Heat of

decomposition/

kJ mol-1

20 190

0 250

-10 360

H2NS

OK

O

O

HNO3

HNO3/H2SO4 HN(NO2)2

KHSO4+N2O+H2O

N2O+HNO3

Scheme 3 Decomposition route of potassium sulfamate during

nitration

Table 2 Heat of decomposition for sulfamates in HNO3/H2SO4

Sulfamate Heat of decomposition/kJ mol-1

20 8C 0 8C

H 160 70

Na 180 240

K 190 250

NH4 140 200

ROS

NH2

O

O

–OS

N+H2R

O

O

Fig. 5 Sulfamate isomers

SHO NH2

O

O

SRO NH2

O

O– + R=Na, K, NH4–+

Fig. 6 Electronic charge directions in sulfamates

Table 3 Dipole moments of sulfamates

Sulfamate Dipole moment/D

H 3.3

Na 12.9

K 16.1

NH4 8.8

Y. Sugie, A. Miyake

123

The heats of decomposition of all sulfamates in HNO3/

AcOH are greater than those in HNO3/H2SO4. In acetic

acid (a weak acid), the production of NO2? is lower than

that in H2SO4. The nitration of sulfamates in acetic acid is

slower than those in H2SO4. As nitration slowly pro-

gresses, the temperature rise is more gradual than it is

with H2SO4. Therefore, nitration in acetic acid progresses

more easily. Decomposition by nitric acid in the presence

of acetic acid is slower than that in H2SO4. Therefore, the

heat of decomposition in acetic acid is larger than that in

H2SO4. In undesired condition, nitration of sulfamate in

acetic acid is more hazardous than that in H2SO4

thermally.

Conclusions

The heats of decomposition of sulfamates under nitration

were investigated using reaction calorimetry. The heat of

decomposition at -10 �C was larger than that at 20 �C in

H2SO4 because the dominant decomposition pathway var-

ied with reaction temperature. The results showed that the

dipole moment of sulfamates affected reactivity toward

nitration and that the acid strength affected nitration; the

heat of decomposition in AcOH was larger than that in

H2SO4.

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Table 4 Heat of decomposition of sulfamates in HNO3/H2SO4 and

HNO3/AcOH

Sulfamate Heat of decomposition/kJ mol-1

H2SO4 AcOH

20 8C 0 8C 20 8C 0 8C

K 190 250 360 330

Na 180 240 350 330

NH4 140 200 440 370

Effects of temperature on nitration of sulfamates

123