High-energy coordination polymers (CPs) exhibiting good catalytic effect on the thermal decomposition of ammonium dinitramide

Article abstract of DOI:10.1016/j.jssc.2017.06.027

High-energy coordination polymers (CPs) not only exhibit good energetic performances but also have a good catalytic effect on the thermal decomposition of energetic materials. In this contribution, two high-energy CPs Cu₂(DNBT)₂(CH₃OH)(H₂O)₃·3H₂O (1) and [Cu₃(DDT)₂(H₂O)₂]_n (2) (H₂DNBT = 3,3′-dinitro-5,5′-bis(1H?1,2,4-triazole and H₃DDT = 4,5-bis(1H-tetrazol-5-yl)-2H-1,2,3-triazole) were synthesized and structurally characterized. Furthermore, 1 was thermos-dehydrated to produce Cu₂(DNBT)₂(CH₃OH)(H₂O)₃ (1a). The thermal decomposition kinetics of 1, 1a and 2 were studied by Kissinger's method and Ozawa's method. Thermal analyses and sensitivity tests show that all compounds exhibit high thermal stability and low sensitivity for external stimuli. Meanwhile, all compounds have large positive enthalpy of formation, which are calculated as being (1067.67 ± 2.62)?kJ?mol^?1 (1), (1464.12 ± 3.12)?kJ?mol^?1 (1a) and (3877.82 ± 2.75)?kJ?mol^?1 (2), respectively. The catalytic effects of 1a and 2 on the thermal decomposition of ammonium dinitramide (ADN) were also investigated.

Full text of DOI:10.1016/j.jssc.2017.06.027

Author’s Accepted Manuscript
High-energy coordination polymers (CPs)
exhibiting good catalytic effect on the thermal
decomposition of ammonium dinitramide
Xin Li, Jing Han, Sheng Zhang, Lianjie Zhai,
Bozhou Wang, Qi Yang, Qing Wei, Gang Xie,
Sanping Chen, Shengli Gao
www.elsevier.com/locate/yjssc
PII:
S0022-4596(17)30248-7
http://dx.doi.org/10.1016/j.jssc.2017.06.027
YJSSC19845
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 26 April 2017
Revised date: 9 June 2017
Accepted date: 26 June 2017
Cite this article as: Xin Li, Jing Han, Sheng Zhang, Lianjie Zhai, Bozhou Wang
Qi Yang, Qing Wei, Gang Xie, Sanping Chen and Shengli Gao, High-energy
coordination polymers (CPs) exhibiting good catalytic effect on the therma
decomposition of ammonium dinitramide, Journal of Solid State Chemistry
http://dx.doi.org/10.1016/j.jssc.2017.06.027
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High-energy coordination polymers (CPs) exhibiting good catalytic
effect on the thermal decomposition of ammonium dinitramide
a1
a1
a,b
c
c
a
Xin Li , Jing Han , Sheng Zhang , Lianjie Zhai , Bozhou Wang , Qi Yang , Qing
a,*
a
a,
a
Wei , Gang Xie , Sanping Chen *, Shengli Gao
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of
Education, College of Chemistry and Materials Science, Northwest University, Xi’an, 710127,
China.
b
c
College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji,
721013, China.
State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research
Institute, Xi’an 710065, China
*
Corresponding author. Prof. Sanping Chen, sanpingchen@126.com
Abstract
High-energy coordination polymers (CPs) not only exhibit good energetic
performances but also have a good catalytic effect on the thermal decomposition of
energetic
materials.
In
this
contribution,
two
high-energy
CPs
Cu (DNBT) (CH OH)(H O) ·3H O (1) and [Cu (DDT) (H O) ] (2) (H DNBT =
2
2
3
2
3
2
3
2
2
2 n
2
3
1
,3′-dinitro-5,5′-bis(1H-1,2,4-triazole and H DDT = 4,5-bis(1H-tetrazol-5-yl)-2H-
3
,2,3-triazole) were synthesized and structurally characterized. Furthermore, 1 was
thermos-dehydrated to produce Cu (DNBT) (CH OH)(H O) (1a). The thermal
2
2
3
2
3
1
Both of authors have an equal attribution.
1

decomposition kinetics of 1, 1a and 2 were studied by Kissinger’s method and
Ozawa’s method. Thermal analyses and sensitivity tests show that all compounds
exhibit high thermal stability and low sensitivity for external stimuli. Meanwhile, all
compounds have large positive enthalpy of formation, which are calculated as being
-1
-1
(
1067.67 ± 2.62) kJ mol (1), (1464.12 ± 3.12) kJ mol (1a) and (3877.82 ± 2.75) kJ
-1
mol (2), respectively. The catalytic effects of 1a and 2 on the thermal decomposition
of ammonium dinitramide (ADN) were also investigated.
Graphic abstract
Keywords: High-energy coordination polymers; Ammonium dinitramide; Thermal
decomposition
1
. Introduction
As an active research topic, the development of more powerful, stable and reliable
energetic materials (EMs) is becoming increasingly important for military and civilian
applications[1-3]. Strategies for the development of the new EMs include the
formation of salts[4-6], introduction of amine functional groups into aromatic
moieties of energetic molecules[7-8], formation of conjugated molecular structures[9-
1
0] or incorporating high energetic units into coordination polymers[11-12]. Therein,
high-energy coordination polymers (CPs) offers the advantage of allowing selective
2

design and tunable energy, sensitivity and thermal stability depending on the nature of
the metal ions or organic linkers[13]. Based on various nitrogen-rich energetic ligands
toward producing abundant coordination modes, massive nitrogen gas and high heats
of formation[14-17], high-energy CPs have become a new generation energetic
materials[18]. One of the most outstanding properties of the high-energy CPs is to
develop catalytic affect to reduce the decomposition peak temperature or increase heat
release of energetic materials, such as 1,3,5-trinitro-1,3,5-triazine (RDX), 1,3,5,7-
tetranitro-1,3,5,7-tetrazocine (HMX), ammonium perchlorate (AP) and so on[19-21].
Ammonium dinitramide (ADN, NH N(NO ) ) has been identified as eco-friendly
4
2 2
oxidizer of solid propellants to replace AP[22-23] because of chlorine-free
combustion products, less smoke and higher specific impulse[24]. As is well known,
the performance of solid propellants is dependent strongly on the characteristics of
thermal decomposition[25]. However, the exothermic process of ADN is very slow
and corresponds to a broad exothermic peak[26-27]. Therefore, it is a significant
attempt to improve the performances of thermal decomposition of ADN though
adding high-energy CPs.
Herein, two high-energy CPs, Cu (DNBT) (CH OH)(H O) ·3H O (1) and
2
2
3
2
3
2
[
(
Cu (DDT) (H O) ] (2) were prepared using 3,3′-dinitro-5,5′-bis(1H-1,2,4-triazole)
3 2 2 2 n
H DNBT)/4,5-bis(1H-tetrazol-5-yl)-2H-1,2,3-triazole (H DDT) as energetic ligand.
2
3
Furthermore, 1 was thermos-dehydrated to produce Cu (DNBT) (CH OH)(H O) (1a).
2
2
3
2
3
The standard molar enthalpy of formation of 1, 1a and 2 were calculated as being
-1
-1
-1
(
1067.67 ± 2.62) kJ mol , (1464.12 ± 3.12) kJ mol and (3877.82 ± 2.75) kJ mol ,
respectively. In addition, the catalytic pyrolysis experiments show that 1a can
effectively reduce the temperature of exothermic peak and accelerate the thermal
decomposition of ADN.
2
. Experimental
Caution: The target compounds are hazardous materials, which may occur
explosion under the certain stimulation. The necessary safety precautions (safety
glasses, face shield and plastic spatula) should be prepared during the experiment,
especially when the compounds are prepared on a large scale.
2
.1. Materials and instruments
All starting materials are commercially available and were used without any
purification. ADN was provided by Xi’an Modern Chemistry Research Institute and
3

the details of synthesis were published in reference[28].
Elemental analyses were carried on a Vario EL III analyzer. IR spectra of solid
1
3
compounds were measured using a Bruker FTIR with KBr pellets. C NMR spectra
were recorded on a 500 MHz (Bruker AV500) nuclear magnetic resonance
spectrometer operating at 300 MHz, by using DMSO-d as solvent. Densities of
6
compounds were measured by using Automatic Density Analyzer (ULTRAPYC
o
1
200e). In the temperature range of 25-500 C, TG measurements were performed on
-1
a STA 449C (Netzsch) using N as the atmosphere with a flow rate of 10 mL min .
2
The sensitivity data were collected using a BAM drop hammer and a BAM friction
tester. The purity of the bulk samples was verified by X-ray powder diffraction
(PXRD) measurements on a RU 200 diffractometer (Rigaku) at 60 kV, 300 mA and
Cu Kα radiation (λ= 1.5406 Å). The constant-volume combustion energies of the
compounds were determined using a precise rotating-bomb calorimeter (RBC-type II)
-4
with an accuracy uncertainty of 4.38×10 [29]. The combustion experiment was
repeated six times and then averaged to get the recorded data.
2
.2. Methods
CDR-4P thermal analyzer (Shanghai Balance Instrument Company) was calibrated
by standard substances and used to measure DSC data using N as the atmosphere
2
-1
with a flow rate of 10 mL min . Nonisothermal parameters were acquired at constant
o
-1
o
heating rates of 2, 5, 8 and 10 C min in the temperature range from 45 to 500 C
with the sample mass of 0.6-0.8 mg. The catalytic effect of thermal decomposition of
o
ADN was tested in the temperature range from 50 to 300 C with a mass ratio of 1:3
(catalyst: ADN). Catalyst and ADN were mixed in an agate mortar and stored in a
desiccator.
2
.3. Synthesis
Based on the literature, 3,3′-dinitro-5,5′-bis(1H-1,2,4-triazole) and dihydrate
(H DNBT·2H O)[30] and hydrated 4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazole
2 2
(
H DDT·H O)[31-32] were prepared. The synthesis processes are shown in scheme 1.
3
2
The details of synthesis are provided in the supporting information.
4

Scheme 1. Synthetic routes for H DNBT·2H O and H DDT·H O.
2
2
3
2
2
.3.1. Synthesis of Cu (DNBT) (CH OH)(H O) ·3H O (1)
2 2 3 2 3 2
Cu (DNBT) (CH OH)(H O) ·3H O was synthesized by the following processes:
2
2
3
2
3
2
H DNBT·2H O (0.1 mmol, 26.2mg) and Cu(NO ) ·3H O (0.2mmol, 48.4 mg) were
2
2
3 2
2
added to a mixed solvent of water and methanol (1:2, 6mL), which were dispersed to
form transparent solution by ultrasonic. The mixed liquor was sealed in a 25 mL
o
Teflon-lined stainless steel autoclave and heated to 90 C for 72 h, and then cooled to
o
-1
room temperature at a rate of 5 C h . The blue block-like crystal of 1 was collected
by filtration, washed with ethanol and dried in air (yield 71.8%, based on
H DNBT·2H O). IR: 3649, 3566, 3462, 3431, 1649, 1543, 1510, 1400, 1338, 1238,
2
2
-
1
1
1
2
141, 846, 790, 711, 659 cm ; elemental analysis (%) calcd for C H N O Cu : C,
9 16 16 15 2
5.11; H, 2.24; N, 31.29; found: C,15.18; H, 2.44; N, 31.32.
.3.2. Synthesis of Cu (DNBT) (CH OH)(H O) (1a)
2
2
3
2
3
1
was put into a drying tubular oven and kept heating at 140 °C for two hours under
N atmosphere. Blue powders of 1a were obtained. IR: 3563, 3470, 3428, 1652, 1533,
2
-1
1
513, 1336, 1238, 1134, 843, 786, 715, 659 cm ; elemental analysis (%) calcd for
C H N O Cu : C, 16.28; H, 1.51; N, 33.86; found: C,16.18; H, 1.55; N, 33.92.
9
10 16 12
2
2
.3.3. Synthesis of [Cu (DDT) (H O) ] (2)
3 2 2 2 n
H DDT·H O (0.05 mmol, 11.2mg) and Cu(NO ) ·3H O (0.1mmol, 24.2 mg) were
3
2
3 2
2
dispersed in water (6 mL) by ultrasonic. The mixed liquor was sealed in a 15 mL
o
Teflon-lined stainless steel autoclave and heated to 180 C for 72 h, and then cooled
o
-1
to room temperature at a rate of 5 C h . The green block-like crystal of 2 was
collected by filtration, washed with water and dried in air (yield 82.5%, based on
H DDT·H O). IR: 3674, 3566, 3417, 1645, 1573, 1533, 1463, 1408, 1342, 1218, 1164,
3
2
-1
1
132, 1004, 927, 842, 769, 754, 702 cm ; elemental analysis (%) calcd for
5

C H N O Cu : C, 15.22; H, 0.63; N, 48.82; found: C,15.18; H, 0.73; N, 48.66.
8
4
22
2
3
2
.4. Crystallographic data collection and refinement
The single-crystal X-ray diffraction data of 1 and 2 were collected by using a
Bruker Smart Apex CCD diffractometer equipped with graphite monochromatized Cu
Kα radiation (λ = 0.71073 Å) using ω and φ scan modes. The single-crystal structure
was solved by direct methods using SHELXS-97[33] and refined by means of full-
2
matrix least-squares procedures on F with SHELXL-97 program. All non-H atoms
were located using subsequent Fourier-difference methods and refined anisotropically.
Other details of crystal data, data collection parameters and refinement statistics are
given in Table 1. Selected bond lengths and bond angles of 1 and 2 are listed in Table
S2 and Table S3.
Table 1 Crystal data and structure refinement details for 1 and 2.
Compound
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
1
2
C H N O Cu
C H N O Cu
3
9
16 16 15
2
8
4
22
2
715.48
630.95
Orthorhombic
Monoclinic
P2 2 2
P2 /c
1
1
1
1
7.701(3)
14.472(6)
21.719(9)
90
8.6924(18)
8.5606(17)
12.798(3)
90
b (Å)
c (Å)
α (°)
β (°)
90
90.075
90
γ (°)
90
3
V (Å )
2420.6(17)
4
952.3(4)
2
Z
F(000)
1440
618
2
GOOF on F
0.994
1.108
R indices (all data)
Final R indices [I>2σ (I)]
CCDC
R = 0.1048, wR = 0.1621
R = 0.0533, wR = 0.0870
1 2
1
2
R = 0.0675, wR = 0.1438
R = 0.0380, wR = 0.0795
1
2
1
2
1521036
1521035
6

3
. Results and discussion
3
.1. Structural description
1
crystallizes in the orthorhombic space group P2 2 2 and the calculated density is
1 1 1
-3
1
.963 g cm . The measured XRPD patterns of 1 and 1a closely matches the simulated
single-crystal diffraction data of 1, indicating a structural maintenance upon guest-
-3
water removal. The measured density of 1a is 1.921 g cm . The asymmetric unit of 1
2
+
2-
contains two crystallographically independent Cu ions, two DNBT ligands, six
water molecules and one methanol molecule. Cu1 is penta-coordinated in tetragonal
2
-
pyramid geometry by four nitrogen atoms (N2, N4, N9 and N12) from DNBT and
one oxygen (O9) atom from coordinated methanol molecule. Cu2 is penta-
coordinated in trigonal bipyramid geometry by two nitrogen atoms (N5 and N13)
2
-
from DNBT and three oxygen atoms (O10, O11 and O12) from three coordinated
2
-
water molecules (Fig. 1a). The DNBT adopts the bidentate chelating and bridging
modes to connect two Cu(II) ions to display a binuclear unit. Furthermore, the
adjacent binuclear units are linked through hydrogen-bonding to form 3D dense
supramolecular structure (Fig. 1b). The detailed hydrogen-bonding information is
shown in Table S4.
2
crystallizes in the monoclinic space group P2 /c, and shows a dense 3D MOF
1
-3
structure. The calculated density is 2.200 g cm , possessing the highest density of the
reported DDT-based energetic compounds[31,32]
.
In the asymmetric unit, there are
-
one and a half crystallographically independent Cu(II) ions, one DDT , and two
coordinated water molecules. Cu1 is coordinated by four nitrogen atoms (N1, N9,
3
-
N1A and N9A) from four DDT and two oxygen atoms (O1 and O1A) from two
coordinated water molecules to form a distorted octahedral geometry (Fig. 2a). The
Cu2 also exists in a distorted octahedral geometry, which is hexa-coordinated by five
3
-
nitrogen atoms (N2, N3, N4, N7 and N8) from DDT and one oxygen atom (O1)
from one coordinated water molecule (Fig. 2b). Furthermore, three Cu(II) ions are
linked by a water bridge and two triazole bridges to from a trinuclear unit (Fig. 2c).
Finally, a 3D framework is constructed by these trinuclear units (Fig. 2d).
7

2
+
Fig. 1. (a) The coordination environment of Cu center of 1, (b) The 3D supramolecular structure
of 1, the purple dashed line behaves hydrogen bond.
Fig. 2. (a) The coordination environment of Cu1, (b) The coordination environment of Cu2, (c)
The trinuclear unit of 2, (d) 3D framework structure of 2.
3
.2. Thermal decomposition
PXRD experiments were carried out on the powder samples of the compounds to
confirm the phase purities of the materials (Figs. S1 and S2). The DSC and
thermogravimetry (TG) curves were used to illustrate the thermal decomposition
o
-1
processes of 1, 1a and 2 with a linear heating rate of 10 C min in N atmosphere.
2
From Fig. 3a, 1 includes two stages of weight loss and an exothermic peak. The first
8

o
o
weight loss is 7.63% from 98 C to 123 C, which is attributed to the loss of three
lattice water molecules (7.54%)[34]. The decomposition of the main framework starts
o
o
at 281.5 C and ends at 383 C. The final product of CuO identified by PXRD with
residue mass percentage of 22.32%, which is in good agreement with the calculated
value of 22.24%. The collapse of the main framework of 1 is rapid with the intense
exothermic peak in DSC curve (Fig. 3b). For 1a, a sudden weight loss is observed
o
o
o
from 182 C to 328.9 C, corresponding to one sharp peak (T = 304.8 C) in the DSC
p
curve. According to PXRD for the residual of 1a, CuO is the final product. 2
undergoes a rapid thermal decomposition process in the temperature range of 210-372
o
C, which corresponds to one sharp exothermic peak in the DSC curve. Finally, 2
completely converts to Cu(CN) [35] with a residue mass percentage of 54.80%,
2
which is in good agreement with the calculated value of 54.96%.
Fig. 3. (a) TG curves of 1, 1a and 2, (b) DSC curves of 1, 1a and 2.
3
.3. Non-isothermal kinetics analysis
The apparent activation energy (E) and the pre-exponential factor (A) were
calculated by Kissinger’s method[36] and Ozawa’s method[37-38], which could be
applied to estimate the thermokinetics stability of 1, 1a and 2. The exothermic peak
o
temperatures were measured from the DSC curve at heating rates of 2, 5, 8, and 10 C
-1
min . The apparent activation energy E and E , pre-exponential factor A and linear
k
o
k
correlation coefficients R and R are shown in Table 2. The Kissinger equation (1)
k
o
and Ozawa equation (2) are as follows, respectively:
9

AR E 1
ln( ) ln
(1)
2
Tp
E
R Tp
0
.4567E
log
C
RTp
(2)
where T is the peak temperature; E is the apparent activation energy; A is the pre-
p
-1
-1
exponential factor; R is the gas constant, 8.314 J mol K ; β is the linear heating rate
and C is constant.
On the basis of the calculated values of E and lnA, the Arrhenius equations can be
a
3
expressed as follows: ln k = 9.8245 – 144.38×10 /(RT) for 1, ln k = 7.3131 –
3
3
1
07.06×10 /(RT) for 1a, and ln k = 10.9145 – 150.18×10 /(RT) for 2.
Table 2 The thermokinetic parameters of the exothermic processes at different heating rates.
Peaks temperatures T (°C)
p
-1
Heating rate β (°C min )
1
1a
2
2
5
8
319.3
337.8
347.2
351.5
267.9
289.2
298.5
304.8
298.2
313.4
321.7
327.5
10
Kissinger’s method
-
1
E (kJ mol )
143.08
9.8245
0.9998
105.23
7.3131
0.9989
149.21
10.9145
0.9982
k
-1
ln A (s )
k
Linear correlation coefficient (Rk)
Ozawa-Doyle’s method
-
1
E (kJ mol )
145.67
108.89
0.9991
151.14
0.9984
o
Linear correlation coefficient (Ro)
0.9998
3
.4. Standard molar enthalpy of formation and sensitivity tests
The standard molar enthalpy of formation ( Hm ) is fundamentally important to
f
evaluate energetic performance of compounds. More positive values lead to
potentially better detonation properties[39]. In this work, the constant volume energy
of combustion (∆ U) of compounds were measured by a RBC-type II typed rotating-
c
1
0

oxygen bomb calorimeter, which were used to calculate the standard molar enthalpy
of formation.
Based on Hess’s law and the combustion reaction:
cHm = ∆
Cu C H N O (s)(1) + 6.5O (g) = 2CuO (s) + 9CO (g) + 8H O (l) + 8N (g) (4)
U + ∆nRT
(3)
c
2
9
16 16 15
2
2
2
2
Cu C H N O (s)(1a) + 6.5O (g) = 2CuO(s) + 9CO (g) + 5H O(l) + 8N (g)
(5)
2
9
10 16 12
2
2
2
2
Cu C H N O (s)(2) + 9.5O (g) = 3CuO(s) + 8CO (g) + 2H O(l) + 11N (g)
(6)
3
8
4
22
2
2
2
2
2
-1
-1
where ∆n is the difference value of molar amount of gases, R = 8.314 J mol K , T
298.15 K.
=
The thermodynamic data of 1, 1a and 2 are listed in Table 3. The standard molar
enthalpy of formation of the combustion products are obtained from NIST chemistry
-1
webbook[40], Hm (CuO, s) = (-156.06) kJ mol , Hm (CO , g) = (-393.51 ± 0.13)
2
f
f
-1
-1
kJ mol and Hm (H O, l) = (-285.83 ± 0.04) kJ mol .
2
f
Table 3 Thermochemical data of 1, 1a and 2.
-1
-1
-1
-1
Compound
∆ U(J g )
∆nRT(kJ mol )
cHm (kJ mol )
Hm (kJ mol )
c
f
1
-10110.74 ± 3.24
-10390.32 ± 4.36
-13405.71 ± 4.03
26.01
26.01
23.54
-7208.02 ± 2.32
-6846.98 ± 2.88
-8065.74 ± 2.54
1067.67 ± 2.62
1464.12 ± 3.12
3877.82 ± 2.75
1a
2
To evaluate the safety of these compounds, impact and friction sensitivities tests
were carried out. The impact sensitivity tests show that all compounds did not fire at
the highest point of 200 cm corresponding to an impact energy of 40 J, and no friction
sensitivity is observed up to 36 kg (360 N). This indicates that these compounds are
insensitive to the impact and friction stimuli. The physicochemical properties of 1, 1a
and 2, and classical energetic materials are listed in Table 4.
Table 4 The physicochemical properties of 1, 1a, 2 and classical energetic materials.
Compound
1
1.953/1.963
31.31
1a
1.921
33.86
304.8
1464.12
>40
2
2.169/2.200
48.82
RDX[41]
1.80
37.84
210
HMX[41] CHP[42]
−
3 a
g
g
ρ (g cm )
1.950
37.84
280
105
7.4
1.948
33.49
194
-
o
b
N (%)
c
Tdec (°C)
351.5
327.5
θ
−1 d
1067.67
3877.82
>40
93
Δ H (kJ mol )
f
m
e
IS (J)
>40
7.5
0.5
-
f
FS (N)
>360
>360
>360
120
-
1
1

a
o
b
c
Experimentally determined density (25 C); Nitrogen content; Temperature of decomposition
d
e
f
g
by DSC; Molar enthalpy of formation; Impact sensitivity; Friction sensitivity; Density from
X-ray measurement.
3
.5. Catalytic effect on thermal decomposition of ADN
In order to avoid the affect of lattice water molecules, the thermos-dehydrated
product 1a was obtained at 140 °C for 2 hours. To illustrate the stability of 1a in the
air, hygroscopic experiments were performed (Table S1). The catalytic effects of 1a
and 2 on the thermal decomposition of ADN were tested by a DSC experiment under
o
-1
the heating rate of 10 C min with a mass ratio of 1:3 (catalyst: ADN). The Ozawa's
method was used to investigate the activation energy (E ) of the thermal
a
decomposition for ADN, 1a+ADN and 2+ADN.
o
As shown in Fig. 4, the pure ADN show a small endothermic peak near 92 C and a
o
broad exothermic peak in the temperature range of 140-238 C, corresponding to
melting and completely decomposition of ADN, respectively[43]. The DSC curves for
decomposition of ADN in addition of 1a or 2 were measured in the same conditions.
There are no obvious change in endothermic peaks, indicating that additives have no
significant impact on the melting process of ADN. But the exothermal peak of ADN
with 1a and 2 show noticeable differences. In the case of 1a used as additives, the
o
o
exothermal peak become more sharp (109.1 C to 170.9 C) and peak temperature is
o
significantly reduced to 148.7 C. The result indicates that 1a can effectively reduce
width of exothermic peak and accelerate the thermal decomposition of ADN. For 2, a
similar DSC curve with the pure ADN indicates that 2 has no catalytic activity. The
activation energy (E ) of the thermal decomposition for ADN, 1a+ADN and 2+ADN
a
-1
-1
are calculated as being 122.28 kJ mol (ADN), 96.11 kJ mol (1a+ADN) and 120.31
-1
kJ mol (2+ADN), respectively (Table 5). In contrast to the structures of 1a and 2, a
possible mechanism is proposed for the thermal decomposition of ADN. According to
+
previous report[44], the first thermal decomposition step of ADN forms NH4 and
-
-
-
-
N(NO ) (DN ), then DN is converted to NO by elimination of N O. However, the
2
2
3
2
-
free DN has highly stabilized due to electron resonance delocalization[45]. In the
catalytic pyrolysis process of ADN, the uncoordinated copper center (penta-
-
coordinated) of 1a with DN may form a coordinated intermediate to weak
-
-
delocalization of DN . As a result, the N-N bond strength of DN is reduced, which is
favorable for the thermal decomposition of ADN[47-49].
1
2

Fig. 4. DSC curves of ADN, 1a+ADN and 2+ADN.
Table 5 The activation energy (E ) of the thermal decomposition for ADN, 1a+ADN and 2+ADN.
a
Peaks temperatures T (°C)
p
-1
Heating rate β (°C min )
ADN
1a+ADN 2+ADN
2
5
8
166.5
126.4
136.8
144.7
148.7
163.3
173.9
180.5
184.7
177.6
183.8
187.9
10
Ozawa-Doyle’s method
122.28
-1
E (kJ mol )
96.11
120.31
0.9973
o
Linear correlation coefficient (Ro)
0.9986
0.9955
Furthermore, the effect of the amount of 1a on the thermal decomposition of ADN
were studied. With increasing the amount of 1a, the exothermal peak temperature and
peak width of ADN decrease rapidly (Fig. 4). The release-heat increase rapidly with
the amount of 1a in the early stage and reaches the maximum value at 7%,
subsequently, with increasing the amount of 1a, the release-heat shows a downward
trend (Table 6). When the amount of 1a is 7%, the release-heat significantly increases,
which indicates that 1a can be used as an efficient catalyst to improve the thermal
decomposition characters of ADN.
1
3

Fig. 5. DSC curves of ADN with different amount of 1a.
Table 6 The onset temperature and release-heat of thermal decomposition of ADN with different
amount of 1a.
Mass percent (%)
0
2
5
7
10
15
20
o
Onset temperature ( C)
141.2 138.7 125.2 119.8 109.1 104.5 103.6
2260 2652 2854 3183 2777 2306 2064
-
1
Heat release (J g )
4
. Conclusion
High-energy CPs, Cu (DNBT) (CH OH)(H O) ·3H O (1, H DNBT = 3,3′-
2
2
3
2
3
2
2
dinitro-5,5′-bis(1H-1,2,4-triazole) and [Cu (DDT) (H O) ] (2, H DDT = 4,5-bis(1H-
3
2
2
2 n
3
tetrazol-5-yl)-2H-1,2,3-triazole), were synthesized and fully characterized. Meanwhile,
was thermo-dehydrated to produce 1a. The remarkably high density of 2 is the
1
highest yet reported for DDT-based energetic compounds. These compounds exhibit
high thermal stability and insensitivity to impact and friction. The standard molar
enthalpy of formation of 1, 1a and 2 were calculated as being (1067.67 ± 2.62) kJ
-1
-1
-1
mol , (1464.12 ± 3.12) kJ mol and (3877.82 ± 2.75) kJ mol , respectively. What’s
more, the catalytic experiments show that 1a can significantly improve the
characteristics of thermal decomposition of ADN.
Supporting information
The synthesis process of 3,3′-dinitro-5,5′-bis(1H-1,2,4-triazole) dihydrate
(
H DNBT·2H O)
and
4,5-bis(1H-tetrazol-5-yl)-2H-1,2,3-triazole
hydrate
2
2
(
H DDT·H O). Hygroscopic experiments of 1a and the weight (g) of 1a after
3
2
different days in air (Table S1). PXRD curves of 1, 1a and 2 (Fig. S1 and Fig. S2), the
selected bond lengths and bond angles of 1 and 2 (Table S2 and Table S3) and the
1
4

hydrogen-bonding lengths and angles for 1 (Table S4). CCDC numbers for 1 and 2,
521036 and 1521035. These data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data
1
request/cifwww.ccdc.cam.ac.uk/data request/cif
Acknowledgments
We gratefully acknowledge financial support from the National Natural Science
Foundation of China (grant no. 21373162, 21673180, 21473135 and 21673181).
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Highlights
1
. Three high-energy coordination polymers (CPs) were synthesized and fully
characterized.
. All compounds have large positive enthalpy of formation, high thermal stability
and low sensitivity for external stimuli.
. The catalytic experiments show that 1a can significantly improve the characteristics
of thermal decomposition of ammonium dinitramide (ADN).
2
3
1
8