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EFFECT OF TEMPERATURE AND TIME VARIATION

ON THE YIELD OF DINITRO PENTAMETHYLENE TETRAMINE
PREPARED FROM HEXAMINE DINITRATE

Omar Al Farouk* and HANY A. El Azab**

*
Faculty of Engineering, Cairo University.
**
Egyptian Armed Forces.
`
ABSTRACT
Preparation of dinitro pentamethylene tetramine (DPT) through
the action of nitrating mixture formed of ammonium nitrate and fuming
nitric acid on hexamine in presence of acetic acid, acetic anhydride and
p-formaldehyde has been carried out. DPT was also prepared from
hexamine dinitrate which is an important intermediate appears through
preparation of DPT from hexamine directly. Prepared DPT was purified
and then characterized through the determination of melting point,
element test, FTIR spectrometry, UV spectrometry, and high
performance liquid chromatography "HPLC". The obtained results were
reliable and consistent with the literature. DPT was prepared at different
temperatures. The variation of some factors like: temperature and time
has been investigated. The obtained results were reliable and
consistent with the literature.

KEY WORDS: Energetic Materials, Hexamine, DPT, Hexamine dinitrate,

Preparation, Characterization, Analysis techniques.

.NOMENCLATURE:

Symbols

ρ Density (g / cm3).
υs Symmetric vibrations.
υas Asymmetric vibrations.
XA Fractional conversion of substance Hexamine Dinitrate.
Y Yield of the DPT.

1. Introduction

Design of future weapon systems requires the use of energetic material

formulations having enhanced performance (energy output) and reduced
vulnerability during manufacturing, handling, storage and transportation.
Several important design considerations for such formulations include

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improved mechanical properties, extended service life and reduced

environmental impact in manufacture, use and disposal [1]. Energetic
materials are substances or mixtures that react chemically to release energy
required for their intended application. Bachmann and Sheehan [2] developed
a method of preparing DPT. This method involved nitrolysis of hexamine with
ammonium nitrate – nitric acid solution and acetic anhydride. The steps of the
formation of DPT can be presented according to the following scheme:-[3,4,5]

Hexamine + Acetic acid + Acetic anhydride 44 ± 1│ C Reaction mixture

+ (Ammonium nitrate / Nitric acid) 15 minutes at end of 1st addition

DPT + Spent nitrolyzing medium

2. Experimental work

Samples were prepared and filtered in a fuming cupboard provided with glass
shelter and air suction system for ventilation. Safety regulations have been
strictly applied. The preparation setup consisted of a flat-bottom 0.5L flask
equipped with a mechanical stirrer, three dropping funnels, and a
thermometer. Filtration of the prepared samples was done using Buchner
funnel - pump system.

2.1 Preparation of hexamine dinitrate

Hexamethylene dinitrate (“hexamine” dinitrate) is an important intermediate in

the pathway of preparation of DPT (Dinitro–Pentamethylene-Tetramine). Its
preparation [6,7] involved the addition of a solution of hexamine (10g,
0.07mol) in distilled water (17.5ml, 0.972mol) drop by drop to nitric acid
(specific gravity =1.4, 11.75ml, 0.261mol) already present in the above-
mentioned setup. The reaction temperature was fixed at 15°C; and the rate of
hexamine solution addition was controlled to meet this condition. Finally, the
mixture was cooled to 5°C and Hexamine dinitrate was separated from the
reaction mixture using a vacuum pump and dried in a vacuum oven.

2.2 Preparation of DPT

2.2.1 Preparation of DPT from hexamine

DPT is an important energetic material [8,9,10]. This preparation was effected

by the introduction of a mixture of p- formaldehyde (0.566g, 0.0188mol),
glacial acetic acid (25ml, 0.437 mol) and acetic anhydride (0.4ml, 0.0042 mol)
into the reaction flask. The following reagents were added simultaneously at
controlled rates over a fifteen minutes period:

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 Solution of hexamine (3.366g, 0.024 mol) in glacial acetic acid (5.248 ml,
0.0917 mol).
 Acetic anhydride (10 ml, 0.106 ml).
 Solution of ammonium nitrate (2.9g, 0.036 mol) in nitric acid (2ml, 0.0476
mol conc. 99% or more).
The reaction temperature was maintained at 44 + 1°C throughout this
procedure. The mixture was left for an additional 15 minutes. The reaction
mixture was then quenched by chilling it to 12°C. Rapid separation of the solid
phase by filtration was then carried out.

2.3 Preparation of DPT from hexamine dinitrate

DPT was also prepared starting from the hexamine dinitrate. To a

mixture formed of glacial acetic acid (5ml, 0.0874mol) and acetic anhydride
(2ml, 0.0212mol), hexamine dinitrate (1g, 0.00375mol) was added as one
portion. Then the reaction mixture was left, for a fifteen minute period, at a
temperature of (44+1°C) which was also maintained throughout this
procedure. The reaction mixture was then quenched by chilling it to 12°C.
Rapid separation of the solid phase was then carried out. The product was
then washed and dried. Concentrations of both the unreacted hexamine
dinitrate and the formed DPT were measured using the Agilent 1100 series
HPLC. The effects of temperature and time were investigated. Reaction
temperature was varied from 15 to 65°C; also reaction time was varied from 0
to 10 hrs. Conversion of hexamine dinitrate into DPT was initially followed up
from zero to 15 minutes at 45°C. This temperature has been recommended
by two different authors [11, 12].Lower and higher temperatures have been
also fixed during execution of the mentioned reactions. Results of the
mentioned follow up are tabulated in tables (5 - 10).

2.4 Characterization of prepared samples

2.4.1 Melting point determination

An “IA – 9100 series” Digital melting point apparatus was used to measure the
melting point of both the standard and the prepared samples. Small amount of
the perfectly dried material was inserted in a capillary tube and placed into the
metallic block which is equipped with a glass window and a digital monitor for
displaying the temperature. For each substance, three consecutive trials were
performed and the average value was calculated.

2.4.2 Elemental Analysis

Carbon, hydrogen and nitrogen were determined using a Perkin-Elmer 2400,

CHN elemental analyzer equipped with AD-4 auto balance. Aluminum
capsules of special design were used. In these capsules, the samples were

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decomposed in presence of oxygen at a temperature of 950 °C. The

mentioned elements were converted into their stable oxides. The combustion
tubes contained chemicals which can absorb the interfering products; while
the remaining products like carbon dioxide, water, nitrogen oxides and
residual oxygen were transferred by means of a stream of helium to the
reduction stage. The reduction tube was packed with copper and it was
maintained at a temperature of 670 °C, in which nitrogen oxides were
reduced. The remaining gaseous products (water and carbon dioxide) were
not reduced but transferred to the stage of absorption. The percentage of
carbon, hydrogen, and nitrogen could therefore be recorded precisely.

2.4.3 FTIR spectrometry

FTIR is one of the most powerful identification spectroscopic techniques. The

IR spectrum acts as a fingerprint for each structure. No two compounds have
the same IR spectrum. This technique is used in qualitative analysis and
identification of most commonly used explosives. A Schimadzu 8100 series
FTIR spectrophotometer was used to obtain the FTIR spectrum for the
prepared samples. The solid sample (2 mg) and the dry KBr (350 mg) were
ground, mixed and pressed in the form of a standard disc using an E-Z press
(12 ton). The selected wave number range used was 400– 4000 cm-
1(appropriate range for most organic compounds). The IR-spectrum was also
obtained for pure KBr for calibration of the instrument. The selected FTIR
operating conditions were as follows:-
Measuring mode: Transmission T %, Resolution: 4.0 cm-1, Detector:
standard, Range: 4000:400 cm-1, Number of scans: 40, and Aperture: auto.

2.4.4 UV spectrometry

UV spectroscopy is one of the most suitable methods in quantitative analysis

especially for dilute solutions. UV detectors are commonly employed in most
chromatographic systems. A Schimadzu UV-vis spectrometer double beam,
UV-1700 has been used to run spectroscopic analysis of the prepared
hexamine dinitrate and DPT by measuring its wave length at maximum
absorption. The selected operating conditions were: Wavelength range: from
600 to 190 nm, Scan speed: medium, Sampling interval: 1 nm, and Measuring
mode: absorbance.

2.4.5 High performance liquid chromatography

High performance liquid chromatography (HPLC) technique can be used as a

qualitative and quantitative method of identification of explosives and other
organic substances through the retention time and the peak area. HPLC can
also be employed not only for separation but also for identification, by
comparing the retention time of the unknown compound to that of a known
one (standard) under the same operating conditions. The instrument in this
work was Agilent 1100 series HPLC instrument. The column was Zorbax

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elipse (XDB-C8, analytical 4.6x150mm, 5-micron) the working temperature

°
was 28 C. The mobile phase was 30 % ACN – 70 % water (HPLC grade).
The UV detector was tunned at 220 nm. The flow rate was 0.5 ml/min., cover
a period of 25 minutes, and the injection volume was 3 ml. Many trials have
been done to obtain chromatograms separated with high resolution. The
mentioned operating conditions were used and separation was done perfectly
except that of hexamine and hexamine dinitrate. It was difficult to separate
them with high resolution since interference took place nearly at the same
retention time.

3. Results

3.1 Yield of prepared compounds

3.1.1 Yield of the hexamine dinitrate

Characteristics of the maximum obtainable hexamine dinitrate were

found and given in Tables shown below. Starting by 10g of hexamine;
about 17g of dry hexamine dinitrate was obtained. The average yield
was therefore about 89% .The yield; according to the published data
is about 95 % [13]. Solubility of a small fraction of the product in the
spent acid may be the main reason of the disagreement.

3.1.2 Yield of the DPT

Characteristics of the prepared DPT were determined and are

presented in Tables (1 - 4). Yield of the prepared DPT was about 65%
but in the absence of para-formaldehyde it was only about 30%.
These results are in good agreement with those found in the literature
[2]. The prepared hexamine dinitrate was also converted into DPT
under the action of acetic anhydride- acetic acid mixture at about 45
ο
C according to the published procedures [2], [10]. After aging the
mixture for 15 minutes, it was cooled down to about 12 οC by adding a
suitable quantity of ice. The product was then filtered and dried under
vacuum. Again, the yield was about 65%.

3.2 Characterization of prepared compounds

3.2.1 Melting point values

Referring to characteristics of the pure compounds, it is clear that the

prepared compounds are of high purity as in table (1).

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Table1 Melting points of the prepared compounds.

Measured Melting point
Compound melting point (from literature) (C) Remarks
(C)
prepared 213 --
DPT Nearly pure DPT crystals
pure -- 213 [14], [15]
Hexamine Nearly pure Hexamine
prepared 160 160
dinitrate dinitrate crystals

3.2.2 Elemental microanalysis results

Comparing with the characteristics of the pure compounds, it is clear from

Table 2 that the prepared compounds are of good quality.

Table 2 Elemental analysis of the prepared compounds.

Compound C( %) H( %) N( % )
Pure 27.06 5.26 31.57
Hexamine dinitrate
Prepared 26.91 5.29 31.745
Pure 27.52 4.58 38.53
DPT
Prepared 27.21 4.536 38.094

3.2.3 FTIR results

Vibrations of organic molecules can be divided into two types; vibrations

associated with the molecule as a whole and vibrations associated with the
present functional groups. Employing one of the modern FTIR spectrometer,
the obtained IR spectrum is usually in the range of 4000 cm-1 to 250cm-1. The
vibrations characterizing the molecule as a whole usually give rise to
absorption bands below 1300 cm-1 These bands are useful for identification of
the unknown molecules by comparing their spectrum to that of authentic
known standard molecules . This region is oftenly called the "fingerprint
region". The vibrations associated with the functional groups give usually
absorption bands above 1500 cm-1. These bands are useful in qualitative
analysis of the unknown organic matter. Two characteristic intense bands are
always associated with the NO2 group. These bands can be attributed to the
symmetric (υs) and asymmetric (υas) vibrations of the mentioned group. For
the prepared DPT, the IR absorption bands found using the 8100 series FTIR
spectrophotometer are presented in Table (3). Fingerprints for the prepared
DPT samples are presented in Figure (1). Analyzing the above mentioned
spectra and referring to the reproducibility of the results, the prepared
compounds could be identified with acceptable precision.

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Fig. (1) IR spectrum of the prepared DPT

Table (3) IR absorption bands for prepared DPT and HMX.

The stretching vibrations of

The Intense The CH stretching
the
Compound NO2 stretching mode
(N–NO2) bond in nitramines
bands (cm –1)
–1
(cm –1)
(cm )

υs 1334.6
1272.9
Prepared
Band at 2923.9
DPT
υas 1525.6
1525.6

3.2.4 UV results

The obtained UV absorption spectra of hexamine dinitrate and DPT in ethanol

are presented in figures (2) and (3) respectively. In the ultraviolet region, the
absorption bands depend on the electronic transitions occurring and also on
the effect of the atomic environment on these transitions. The obtained
absorbance regions and also the values of the maximum absorbance wave
length are given in Table (4). Since λmax of Hexamine dinitrate is very close to
that of the DPT when ethanol is employed as a solvent, from these results, it
is clear that identification of the prepared compounds via their UV spectra is
somewhat difficult.

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Fig. (2) UV spectrum of hexamine dinitrate in ethanol

Fig. (3) UV spectrum of DPT in ethanol

Table (4) UV Absorbance regions and maximum absorbance for the prepared
compounds
Maximum
Absorbance
No. Maximum absorbance absorbance
Compounds region (nm)
“λmax”(nm) in methanol “λmax”(nm)
(from literature)
in ethanol

Prepared
1 222 204 200 to 240 [16]
Hexamine dinitrate

2 Prepared DPT 248 203 200 to 250[16]

3.3 Results of HPLC analysis

The following figure, figures (4 -11) are representing the results obtained by
HPLC analysis. The above – mentioned figures show the chromatograms
obtained at the end of the investigated reaction time. By examining the
obtained chromatograms, the prepared compounds were identified
qualitatively and quantitatively to calculate the conversion and yield
percentage as in tables (11 - 14).

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Fig. (4) HPLC Chromatogram of standard hexamine

Fig. (5) HPLC Chromatogram of prepared hexamine dinitrate

Fig. (6) HPLC Chromatogram of standard acetone

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Fig. (7) HPLC Chromatogram of prepared DPT

3.2.1 Interpretation of the results obtained at 15 οC

The reaction is generally very slow. 600 minutes were sufficient to

convert about 40% only from hexamine dinitrate into DPT.

Table (5) DPT and hexamine dinitrate concentrations and concentration terms
at different reaction times at 15 C.
Time
0 5 10 15 120 240 360 480 600
(min.)
Hexamine
dinitrate 0.592695 0.58921 0.58701 0.58359 0.54164 0.48039 0.44791 0.38552 0.35588
(mg / ml )
DPT
0 0.00152 0.00280 0.00509 0.03200 0.07230 0.10200 0.12600 0.13500
(mg / ml )

3.2.2 Interpretation of the results obtained at 25 οC

The rate of conversion of hexamine dinitrate into DPT at 25 οC was

found relatively higher than that at 15 οC. Depletion of about 60% of hexamine
dinitrate was achieved after 600 minutes.

Table (6) DPT and hexamine dinitrate concentrations and concentration terms
at different reaction times at 25 C.
Time
0 5 10 15 120 240 360 480 600
(min.)

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Hexamine
dinitrate 0.59780 0.58660 0.58502 0.58129 0.48863 0.40006 0.32762 0.29629 0.23547
(mg / ml )
DPT
0 0.00593 0.00749 0.00985 0.08700 0.16200 0.21500 0.23100 0.23500
(mg / ml )

3.2.3 Interpretation of the results obtained at 35 οC

The reaction rate was appreciably higher than that found below this
temperature. About 90% conversion was recorded after 600 minutes.

Table (7) DPT and hexamine dinitrate concentrations and concentration terms
at different reaction times at 35 C.
Time 0 5 10 15 120 240 360 480 600
(min.)
Hexamine 0.58809 0.53282 0.52159 0.45800 0.3694 0.20273 0.1340 0.08620 0.05255
dinitrate
(mg / ml )
DPT 0 0.03425 0.05046 0.10219 0.1785 0.31534 0.3684 0.40172 0.43891
(mg / ml )

3.2.4 Interpretation of the results obtained at 45 οC

Decomposition of hexamine dinitrate became more and more faster

than that found below this temperature. About two thirds of the hexamine
dinitrate were depleted during the first fifeteen minutes. This temperature has
been already recommended by many authors [12, 13].

Table (8) DPT and hexamine dinitrate concentrations and concentration terms
at different reaction times at 45 C.

Time
0 1 2 3 5 6 8 10 12 15
(min.)

Hexamine
dinitrate 0.5982 0.56009 0.54648 0.5245 0.4456 0.40834 0.35321 0.33597 0.28897 0.2048
(mg / ml )
DPT
0 0.02540 0.03780 0.0591 0.1250 0.14949 0.19645 0.20519 0.25267 0.3215
(mg / ml )

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3.2.5 Interpretation of the results obtained at 55 οC

The concentrations and concentration terms found at 55 C were

neither fitting tightly to the first order nor to the second order ordinary kinetic
models. The reaction at this temperature was more and more fast. About 74%
of the hexamine dinitrate were depleted during the first fifteen minutes.
The formed DPT was somewhat less than the theoretical. This may be
attributed to some sort of the side reactions. This may also explain the
recommendation given by many authors which limit the reaction temperature
to 45 C.

Table (9) DPT and hexamine dinitrate concentrations and concentration terms
at different reaction times at 55 C.
Time
0 5 10 15
(min.)
Hexamine dinitrate
0.59698 0.40825 0.32540 0.15522
(mg / ml )
DPT
0 0.13400 0.18750 0.27640
(mg / ml )

3.2.6 Interpretation of the results obtained at 65 οC

The concentrations and concentration terms found at 65 C were

neither fitting tightly to the first order nor to the second order ordinary kinetic
models. The reaction was as usual more and more fast. About 77% of the
hexamine dinitrate were depleted in the first fifteen minutes. The DPT yield
was less than that found in the literature.

Table (10) DPT and hexamine dinitrate concentrations and concentration

terms at different reaction times at 65 C.
Time
0 5 10 15
(min.)

Hexamine dinitrate
0.59780 0.39850 0.28370 0.14048
(mg / ml )

DPT
0 0.14200 0.19230 0.26235
(mg / ml )

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The fractional conversion ''XA'' of hexamine dinitrate and the

yield ''Y'' of DPT were calculated at each investigated reaction time.
For easy comparison, the results of conversion and yield at different
reaction times for all temperatures were collected in tables (11-14);
and figures (8 -11).

X A (% )
''Conve rsion of DP T a t XA (% ) at 35°C
diffe re nt re a ction tim e s XA (% )at 15°C
a t 15,25,35 C'' XA (% ) at 25°C
1

0.8

0.6

0.4

0.2

0
1 2 3 4 5 6 7 8 9
Tim e (m in.)

Fig. (8) Conversion of hexamine dinitrate versus time

between 15 and 35 °C.

Table (11) Calculated results of hexamine dinitrate conversion at different

reaction times and at tested temperatures.
Point
number 1 2 3 4 5 6 7 8 9
t(min.) 0 5 10 15 120 240 360 480 600
XA (%)at
15°C 0 0.5879 0.959 1.536 8.614 18.948 24.428 34.954 39.955
XA (%) at
25°C 0 1.87 2.137 2.761 18.26 33.078 45.1957 50.436 60.61
XA (%) at
35°C 0 9.399 11.308 22.12 37.187 65.527 77.21 85.3425 91.0633
XA(% )

''Conve rsion of DPT XA(% ) at 45°C

a t diffe re nt re a ction tim e s XA(% ) at 55°C
a t 45,55,65 C''
XA(% ) at 65°C
1

0.8

0.6

0.4

0.2

0
1 2 3 4
Tim e (m in.)

Fig. (9) Conversion of hexamine dinitrate versus time

between 45 and 65 °C.

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Table (12) Calculated results of hexamine dinitrate conversion at different

reaction times and at tested temperatures.
Point number 1 2 3 4
Time 0 5 10 15
XA (%) at 45°C 0 25.502 43.836 65.76
XA (%) at 55°C 0 31.614 45.492 74
XA (%) at 65°C 0 33.338 52.54 76.5
Y ield(% )

''Yie ld of DP T Y ield(15°C)
a t diffe re nt re a ction tim e s Y ield(25°C)
a t 15,25,35 C''
Y ield(35°C)
120

100

80

60

40

20

0
1 2 3 4 5 6 7 8 9
Tim e (m in.)

Fig. (10) Yield of DPT versus time between 15 and 35 °C.

Table (13) Calculated results of DPT yield at different reaction times and at
tested temperatures.
Point
number 1 2 3 4 5 6 7 8 9
t(hour) 0 5 10 15 120 240 360 480 600
Yield(15°C) 0 53.2 59.987 66.97 76.47 78.55 85.96 74.209 69.557

Yield(25°C) 0 64.582 71.56 72.81 97.239 100.2713 97.098 93.48 79.1386

Yield(35°C) 0 75.6 92.58 95.846 99.591 99.846 98.996 97.66 100.0039
Y(%)

''Yield of DPT Yield(45°C)

at different reaction times Yield(55°C)
at 45,55,65 C'' Yield(65°C)
120

100

80

60

40

20

0
1 2 3 4
Time (min.)

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Fig. (11) Yield of DPT versus time between 45 and 65 °C.

Table (14) Calculated results of DPT yield at different reaction times and at
tested temperatures.
Point number 1 2 3 4
t(hour) 0 5 10 15
Yield(45°C) 0 99.98 95.477 99.71
Yield(55°C) 0 86.634 84.241 76.34
Yield(65°C) 0 86.93 74.7 70

4. Conclusion

The yield of the DPT prepared was about 65%; while the yield of the hexamine
dinitrate prepared was about 89%. The analysis of the prepared samples using
an efficient HPLC was a very reliable procedure.
Decomposition of hexamine dinitrate became more and more faster than that
found below this temperature. About two thirds of the hexamine dinitrate were
depleted during the first fifeteen minutes. This temperature has been already
recommended by many authors. The temperature of 45 °C is the optimum
temperature that gives the optimum conversion and yield.

References
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[8] L. Silberman, [U.S. Dept. of the army], U.S patent 2941994, [1960].
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[12] Soloman, Silberman, [U.S. Dept. of the army], "Process for preparing
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[13] T. Urbaniski, “chemistry and technology of explosives, pergaman press

vol. (3), p.117, hetero cyclic nitramines, Warso Poland, 1985.
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[14] X. Heming , T. Zehua and L. Yue , “ theoretical studies on conformation of
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