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Current Organic Chemistry, 2016, 20, 2514-2550

REVIEW ARTICLE

Pyrolysis of Energetic Ionic Salts Based on Nitrogen-rich Heterocycles

Neeraj Kumbhakarna and Arindrajit Chowdhury*

Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai, 40076, India

A R T I C L E H I S T O R Y

Received: February 29, 2016 Revised: April 04, 2016 Accepted: May 13, 2016

DOI: 10.2174/1385272820666160525121523

Abstract: Energetic ionic salts based on nitrogen-rich heterocycles are expected to usher in

a new era in the fields of propellants, explosives, and pyrotechnics owing to their excellent

combustion characteristics, green nature, and the ability to tailor them based on require-

ments. The review focuses on an important aspect of these compounds, other than the syn-

thesis procedure and physico-chemical characterisation, that is frequently overlooked, i.e. the

decomposition pathways and the associated chemical kinetic parameters, which are essential

to elucidate and simulate their combustion characteristics. The reaction mechanisms of four

major families of energetic ionic salts, explored by various experimental and numerical

techniques, are reported in detail.

Keywords: Pyrolysis, ionic salts, nitrogen-rich, heterocycles.

1. INTRODUCTION

In the past two decades, research on ionic salts as possible re-placements of existing energetic compounds as monopropellants, fuels in bipropellant systems, pyrotechnics, and explosives has received a tremendous thrust [1-6]. Energetic compounds, such as explosives, propellants, and pyrotechnics, are typically defined by their ability to generate hot gases by uncontrolled or controlled combustion. Explosives rely on uncontrolled detonation to generate pressure waves, propellants rely on controlled deflagration to de-velop high pressures to propel devices, while pyrotechnics rely on self-sustained deflagraration to generate audio-visual effects. Typi-cal nitro-based explosives, such as 2,4,6-trinitrotoluene (TNT), 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 1,3,5-trinitro-1,3,5-triazine (RDX), as well as the energetic powerhouse 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) are expected to leave pollutants in their wake, contaminating ground water and soil [7, 8]. The commonly used monopropellants and bipropellants from the hydrazine family, such as hydrazine, monomethylhydra-zine (MMH), and unsymmetrical dimethylhydrazine (UDMH), are all known carcinogens, with high levels of toxicity and volatility, rendering them cumbersome to handle. Hence, ionic salts, with several potential advantages, have emerged as the frontrunners to replace the existing class of energetic materials, and provide vari-ous lucrative advantages, such as low handling hazards, environ-mental safety, insensitivity to stimuli, and ability to tailor energetic performance [1, 4-6, 9].

Energetic ionic salts are a class of compounds formed by pair-ing bulky cations with inorganic anions or recently discovered or-ganic anions. Ionic liquids (ILs) are a unique class of ionic salts with melting points below the boiling point of water. Among ionic

*Address correspondence to this author at the Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai, 40076, India; Tel: 022-2576-7504; E-mail: [email protected]

liquids, room temperature ionic liquids (RTILs) are a subcategory of ionic liquids with melting points below the ambient temperature. While the first room-temperature energetic ionic liquid, [CH3CH2NH3]-[NO3], with a melting point of only 12°C was re-ported in 1914 by Walden [10], the modern era of ionic liquids started with the synthesis of 1-butylpyridinium chloride-aluminum(III) chloride mixture by Gale et al. [11]. The reducible nature of N-alkyl-pyridinium cations in basic solutions led to a search for more stable cations. Wilkes [12] studied a wide range of heterocyclic cations with quaternary N-atoms and found the dialkyl-imidazolium cation to be the most suitable one. Further studies by Fannin et al. [13] revealed that the 1-ethyl-3-methyl-imidazolium cation was an excellent compromise between ease of synthesis and desirable properties. However, chloroaluminate ionic liquids, pre-pared by mixing aluminium chloride with dialkyl-imidazolium chlorides, were found to be hygroscopic. This led to the search for anions that would yield ionic liquids stable towards hydrolysis at room temperature, culminating in the discovery of tetrafluorobo-rate, hexafluorophosphate, nitrate, sulfate and acetate salts by Wilkes and Zaworotko [14].

The last two decades have sparked a significant interest in syn-thesis and analysis of new energetic ionic salts initiated by the in-depth study of hydrazinium azides by Klapötke et al. [15-19]. In the last two decades, several research groups, led by Klapötke et al.,Drake et al. and Shreeve et al. have synthesized a wide range of energetic ionic salts, among which a significant portion were liq-uids, with varying properties and applicability as propellants and explosives. A thorough review of the synthesis procedures and the determination of various thermo-physical properties such as den-sity, viscosity (as liquids), melting point, glass transition tempera-ture, decomposition onset temperature, enthalpy of formation, etc., as well as various energetic properties such as oxygen balance, specific impulse, detonation pressure, detonation velocity, impact sensitivity, friction sensitivity, electrostatic discharge sensitivity, ignition delay, etc. was conducted in the recent past [1, 4-6]. The

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Pyrolysis of Energetic Ionic Salts Based on Nitrogen-rich Heterocycles Current Organic Chemistry, 2016, Vol. 20, No. 23 2515

typical families of compounds that have gained prominence are formed by pairing nitrogen-rich cations, such as imidazolium, pyra-zolium, triazolium, tetrazolium, guanidinium, triazinium, or hy-drazinium with oxygen-rich anions such as nitrate, metallic nitrate complexes such as cerium nitrate, lanthanide nitrate, dinitramide, nitrocyanamide, nitrocyanomethanide, and perchlorate as well as with nitrogen-rich anions such as azide, dicyanamide, picrates, imidazolates, triazolates and tetrazolates, and finally with boron-based anions such as borohydride, cyanoborate, dicyanoborate, cyanoborohydride etc.

Besides their conventional application in various branches of science and engineering primarily as solvents, electrolytes, and catalysts, the major motivation behind the current study of energetic ionic salts involve their application in explosive systems, a multi-tude of propulsion systems such as space-craft and aircraft propul-sion, pyrotechnics, as well as in gas generation systems. Advanta-geous properties of these salts include negligible vapor pressure, leading to minimal inhalation hazard; high density, leading to com-pact designs; high thermal stability and reduced sensitivity to im-pact, friction and shock, low corrosiveness, low toxicity, leading to ease of handling and storability; ease of synthesis; and finally, the ability to tailor compounds in order to satisfy various property re-quirements. Though initial focus on energetic ionic salts was con-centrated on development of monopropellants, it has been partially shifted to bipropellant systems with the recent discovery of the hypergolicity of a series of ionic compounds based on various cy-anamide and boron-based anions.

But before these fuels can be employed efficiently and safely in a practical setting, critical performance parameters must be calcu-lated and optimized to match existing propellants. The initial stud-ies on the performance parameters may be concentrated on thermo-chemical calculations utilizing equilibrium-based models to deter-mine the specific impulse, combustion chamber temperature, prod-uct distribution, heat of decomposition or combustion etc. as mono-propellants and bipropellants; as well as utilizing semi-empirical approaches or equilibrium-based models to determine the detona-tion pressure and detonation velocity as explosives. The experimen-tal determination of the thermal stability of the compounds may be carried out by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), coupled with gas analysis techniques, such as mass spectrometry (MS), Fourier transform infrared (FTIR) spectrometry, or gas chromatography. However, the processes in-volved in the practical utilization of the compounds, either during combustion, involving extremely high heating rates, or during stor-age and handling, involving low heating rates, are dependent on the rates of the reactions. The net energy released, the resultant product distributions during these processes and resultant performance pa-rameters are controlled primarily or in part by the chemical kinetics of decomposition in the gas phase as well as condensed phase. Al-though there is considerable understanding of the gas phase reac-tions and their rate parameters, the condensed phase reactions that are crucial in generating the gas phase species that lead to ignition and combustion, are relatively unexplored owing to their complex-ity. Hence, a detailed understanding about the condensed phase reaction pathways and the associated chemical kinetic parameters of their thermal decomposition under low and high heating rates, as encountered during storage and propulsion respectively, is essential. Such data may also be utilized in improving the predictive capabil-ity of numerical models that are used to simulate storability and various combustion phenomena. The review focuses on elucidating the thermal decomposition pathways of several such families of

compounds and presents a comprehensive understanding of the current status of research on pyrolysis of energetic ionic salts. The review begins with discussion of azotetrazolate-based salts and then proceeds to cover imidazolium, triazolium, and tetrazolium-based salts.

2. EXPERIMENTAL AND COMPUTATIONAL TECH-NIQUES

The experimental techniques typically utilised to study the thermolysis of energetic compounds may be classified according to the heating rates achieved during the process. The slow heating rate processes are traditional methods such as thermogravimetric analy-sis (TGA), differential thermal analysis (DTA), differential scan-ning calorimetry (DSC), heating stages, etc. The processes involv-ing fast heating rates are temperature-jump (T-jump) or flash pyro-lysis, confined rapid thermolysis, laser ablation, etc. The diagnostic tools employed to detected the products are typically Fourier trans-form infrared (FTIR) spectroscopy, mass spectrometry (MS), gas chromatography (GC), vacuum ultraviolet photoionization time of flight MS (VUV-PI-TOFMS), etc.

In the computational domain, for analysis of decomposition of compounds, quantum mechanics based calculations are emerging as an effective avenue for corroborating experimentally measured data and providing information otherwise unavailable experimentally. Typically, in such calculations, various levels of theory are com-monly used for molecular structure optimization and frequency calculations. Some of them are B3LYP/6-31G(d) [20, 21], B3LYP/6-311++G(d,p) [22]. CBS-QB3 [23], MP2/6-311++G(d,p) [24], and G4(MP2) [25]. Although it has not become the norm yet, some researchers also resort to IRC (intrinsic reaction coordinate) calculations [26, 27] to ascertain that their transition states indeed connect the respective reactants to the corresponding products. Utilities such as the polarizable continuum model (PCM) [28, 29] are used to model liquid-phase reactions. This model accounts for the continuum solvation effects.

3. DECOMPOSITION OF GUANIDINIUM-BASED IONIC COMPOUNDS WITH NITRATE ANION

Guanidinium and amino-substituted guanidinium form a group of nitrogen-rich cations that have typically been paired with nitro-gen-rich heterocylic anions, namely the azotetrazolate anion, to generate a family of compounds with excellent energetic properties [1, 30-33]. Several works in the past have analysed ionic com-pounds [6, 33-41]. Among these compounds, triaminoguanidinium azotetrazolate (TAGzT) has been found to be one of the most prom-ising gas generants and burn-rate modifiers. The burn rate of pure TAGzT was found to be approximately ten times that of pure RDX, and a mixture of 80% RDX and 20% TAGzT was found to burn 60% faster than pure RDX [42, 43]. Since the combustion charac-teristics of these guanidinium-based compounds have been studied by experimental [31, 36, 38-39, 44-46] and numerical techniques [47-50], the necessity of a detailed chemical kinetic mechanism in the condensed phase was established. Hence these compounds are analysed first, compared to the other families of energetic ionic salts. In this section the data available in the literature on pyrolysis of such ionic compounds which can potentially act as high energy density materials (HEDMs) is reviewed. Although a considerable amount of literature is available on these compounds, only a small fraction of the available works focus on pyrolysis and decomposi-tion mechanisms.

2516 Current Organic Chemistry, 2016, Vol. 20, No. 23 Kumbhakarna and Chowdhury

Research on guanidinium based compounds started gathering momentum in the 1980s with the objective of exploring their usage in propellant, explosive, and pyrotechnic applications. Compounds such as guanidinium nitrate (GN), and triaminoguanidinium nitrate (TAGN), in spite of not having a heterocyclic ring in their molecu-lar structure, may be considered as precursors in the study of pyro-lysis of energetic ionic salts based on nitrogen-rich heterocycles. The decomposition of nitrate-based compounds is discussed ini-tially to determine the possibility of decomposition of the cation and its effect on a simpler energetic anion. One of the earliest works on GN was that by Udupa [34], in which he reported that the com-pound decomposes at around 100 °C on thermolysis. He also dis-cussed the results of thermogravimetric analysis (TGA), differential thermal analysis (DTA) and mass spectrometry (MS) studies of GN. From the TGA data, Udupa found the activation energy to be 192 kJ/mol using the Coats and Redfern method. From his MS results, he proposed that the decomposition process occurs through a proton transfer mechanism. The compound undergoes decomposi-tion into neutral entities which are then vaporised and ionised. Ow-ing to the high ionization energies, Fragmentation also occurs in the mass spectrometer and cyanamide (m/z=42), its dimer dicyandia-mide (m/z=84) and trimer melamine (m/z=126) appear as by-products.

Rapid scan FTIR examination of TAGN by Oyumi and Brill [44] is one of the major works focussing on solid phase transitions and pyrolysis. They reported that thermolysis at high heating rates (>70 K/s) of TAGN at 15 psi (Ar) liberates predominantly HNO3(g) and NH3(g) in the first stage of decomposition. Reaction of HNO3

with the residue from the cation decomposition in the second stage produces NO2, N2O, and HCN. NO2 being a strong oxidiser, even-tually orchestrates the formation of NO. HCN(g) is one of the early carbon-containing products but is short lived because of its high reactivity. Products of the third stage of TAGN decomposition are CO2, NO, H2O, and possibly N2. Nitrogen, being symmetric, can-not be detected by IR spectroscopy, although it is a possible by-product. Under the conditions in Oyumi and Brill’s experiments, the third stage products are more stable than the previous ones. As heating rates increase the distinction between second and third stages becomes more apparent. With increase in pressure the set of pyrolysis products obtained is not very different, but the second and third stages become indistinguishable. Further increase in pressure to 65 psi and above results in very different thermolysis product distribution, characterised by significant increase in HCN concen-tration and decrease in NH3. Also HNO3 is not detected and the presence of carbon oxidation products such as CO and CO2 is prominent. At even higher pressures (>200 psi Ar) concentration of IR-active nitrogen containing species relative to the carbon contain-ing ones decreases further. At high pressures the thermolysis prod-ucts tend to follow chemical pathways that are generally observed in deflagration situations. Because of the nature of the intermediates that TAGN generates, it is capable of enhancing the rate of decom-position of nitramines when mixed with them.

Diaminoguanidinium nitrate (DAGN) and TAGN were studied by Naidu et al. [35] with the motivation that these energetic amine nitrates have a high specific impulse at low isochoric flame tem-perature. Their molecular structure, which apart from high-nitrogen content, has HNO3 as the oxidizer, is responsible for these attrib-utes. The compounds are also capable of producing relatively low molecular weight gaseous decomposition species, thus enhancing the specific impulse. Techniques used by them were TGA, DTA, IR spectroscopy and hot stage microscopy. They claim that the rupture

of the N-N bond in the guanidinium cation (Gu+) is the primary step in thermal decomposition of DAGN and TAGN based on their TGA mass loss profiles and sequential disappearance of bands in the condensed phase IR spectra. From the TGA results they calcu-lated the global rate parameters of both the compounds. For DAGN they report Ea=130 kJ/mol and logA=11.4, and for TAGN; Ea=159 kJ/mol and logA=16. Naidu et al. also studied the effect of several additives on the decomposition of DAGN and TAGN. They found from DTA results that the additives enhance the decomposition of DAGN but TAGN remains unaffected.

GN has remained a compound of interest till recent times be-cause of its excellent explosive properties, and is currently used in the military and commercial sectors. This was the motivation for Oxley et al. to study it along with ammonium nitrate (AN) and urea nitrate (UN) [40]. They carried out DSC analysis of the compounds and extracted Arrhenius activation energies and pre-exponential factors for each of them. Isothermal decomposition products were identified by using liquid chromatography (LC) and gas chromatog-raphy (GC). In these experiments, the temperature set for GN was 250 °C and that for UN was 200 °C. Oxley et al. detected con-densed phase as well as gas phase decomposition products of both the compounds and formulated reaction schemes accordingly. They proposed two mechanisms for AN decomposition. The dominant mechanism depends on temperature but the initial step is dissocia-tion of An to ammonia and nitric acid (Fig. 1, reaction 1). The next step is decomposition of nitric acid to form NO2 (at high tempera-ture) or NO2

+ (below 270°C). This was considered to be the rate-determining step. Thereafter, attack of NO2 or NO2

+on ammonia forms nitrous oxide and water (Fig. 1, reactions 2-4). UN and GN primarily produce gases but leave behind a condensed-phase resi-due. UN can decompose either via dissociation (Fig. 1, reactions 5-7) to produce urea and nitric acid or via dehydration to produce nitrourea (Fig. 1, reaction 8) with subsequent decomposition into gaseous products (Fig. 1, reactions 8-9). The reported heat of reac-tion for the UN dissociation scheme is 121 kJ/mol, and that of the dehydration scheme is �37 kJ/mol. Thus, the route proceeding through reactions 5 to 7 only becomes exothermic when completed through reactions 2, 3, and 4 achieving the final gaseous products. Focusing on GN, Oxley et al. further postulated that its decomposi-tion occurs through three pathways. The major pathway predicts that GN dissociates to nitric acid (Fig. 1, reaction 10) following a pathway analogous to that proposed for AN and UN. Cyanamide (NH2CN) escaping from GN dehydration may either react further in the condensed phase (Fig. 2) or may react with NO2

+ (Fig. 1, reac-tion 12). Another species that GN can form on dehydration is ni-troguanidine (Fig. 1, reaction 13). Subsequent reactions would give cyanamide and nitramine, which rapidly undergo further decompo-sition (Fig. 1, reactions 12, 14, 15 and Fig. 2). An alternate pathway is through the formation of AN as an intermediate in GN decompo-sition (Fig. 1, reaction 16). AN would go through steps 2, 3, and 4 (Fig. 1) as discussed before and cyanamide would dimerize or trimerize (Fig. 2). A fourth route of GN decomposition via urea (Fig. 1, reaction 17) was ruled out by Oxley et al. because neither urea nor its decomposition products, cyanuric acid and biuret, were observed. Overall the decomposition rate of GN was notably slower compared the decomposition rates of UN and AN, which were found to be comparable. This comes as a surprise because the DSC exotherm for UN appears at a much lower temperature (172 °C) than the AN exotherm (327 °C). All three salts undergo an endo-thermic dissociation into nitric acid primarily and the corresponding base, which cools the nitrate salts. Exothermicity comes from sub-sequent reaction involving the produced nitric acid.


To highlight the suitability of GN and TAGN as constituents of gun-propellant formulations owing to their low flame temperatures and high burn-rates, Damse et al. [30] analysed them using TGA-

DTA, FTIR spectroscopy, MS, GC and closed-vessel evaluation. They found in their TGA tests that GN does not evaporate as rap-idly as TAGN in spite of their similar structures. Based on the ob-

+ HNO3

+ HX + O + NO2+ rds

+ NO2+ +

O + H3+

- + H3+ O where X- = HO-

Scheme 1. AN dissociation

[NH2 ]+ - O=C(NH2)2 + HNO3

(AN route: followed by rx 2, 3 & 4)

O=C(NH2)2 + HCNO rds

+ HCNO

Scheme 2. UN dissociation

[NH2 ]+ - + H2O nitrourea/ammonia route

+ N2O + CO2

Scheme 3. UN dehydration

[HN=C(NH2)NH3]+ -

HN=C(NH2)2 + HNO3

HN=C(NH2)2 NC(NH2) + NH3

Scheme 5. GN dissociation

HN=C(NH2)2 + NO2+ HN=C(NH2)NHNO2 + H+

[HN=C(NH2)NH3]+ - HN=C(NH2)NHNO2 + H2

HN=C(NH2)NHNO2 + NH2 rds

O + H2

Scheme 6. GN dehydration

[HN=C(NH2)NH3]+ -

+ NH4

HN=C(NH2)NHNO2 O=C(NH2)2 + N2

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

NH4NO3 NH3

HNO3 H2ONO2 H2

NH3 NH3NO2 N2 O

HO O 2H2

C(OH)NH2 NO3

NH3

NH3 NH4CNO

CONH3 NO3 NH2CONHNO2

NH2CONHNO2 NH3

NO3

NO3 O

NCNH2 NO2

NH2NO2 N2 O

NO3 NCNH2 NO3

O

Fig. (1). Proposed gas-phase decomposition pathways for AN, GN and UN (Oxley et al. [40]).

2 O=C(NH2)2 + NH3

O=C(NH2)2 + HNCO

6 O=C(NH2)2 + 6 HCNO

+ 6 NH3 + 3 CO2

O=C(NH2)2 - H2 [N(CNH2)]3

2 N=C(NH2) C(=NH)(NHCN)

3 N=C(NH2)

+ HNCO

O=C(NH2)2 + HNCO O=C(NH2)(NHCN) + H2

biuret

biuret

cyanic acid

melamine

melamine

cyanoguanidine

melamine

cyanuric acid

cyanuric acid

cyanourea

Scheme 4. UN decomposition products form observed condensed-phase species

NH2C(O)NHC(O)NH2

NH2CONHCONH2

6 NH3

6 HCNO C3H6N6

O NH2CN

NH2

C3H6N6

NH2C(O)NHC(O)NH2 C3(OH)3N3

3 HNCO O3C3(NH)3

O

Fig. (2). Proposed consensed-phase decomposition pathways for UN (Oxley et al. [40]).


servations in the mass loss data from TGA, observed frequencies in FTIR spectra and species detected in MS and GC, Damse et al.proposed reaction schemes for both the compounds. As shown in Fig. 3 they inferred that in most part, heat is liberated by breakage of N-NH2 bonds available in the molecular structure of TAGN and subsequent reactions rather than the dissociation of the oxidizer fragment HNO3 attached by an ionic bond. Energetics of the triaminoguanidinium salts are typically represented by rapid exothermic reactions observed in the decomposition process which occurs immediately after the endothermic melting. The weakest chemical bond available in the molecular skeleton of TAGN is the N-N bond (159 kJ/mol). Hence the initial bond breakage was expected to take place by the homolytic fission of N-NH2 bonds producing the highly reactive NH2 radicals. They dissociate providing the early-stage energy (104.3 kJ/mol), contributing towards rapid combustion. GN on the other hand, is found to undergo slow exothermic decomposition due to absence of the facile N-NH2 bonds in its molecular structure.

C N

NH

NH

H2N

H2N

NH2

- 3NH2 (exo)

1st stage(homolysis)

C NCH4

HN

HN

2nd stage (endo)

endothermicdecomposition

of C-N bond

gas phase reactions:

+ 1/2 N2

C NH

H2N

H2N

(endo)

1st stage

endothermicdecomposition

of C-N bond

TAGN

HNO3

3 NH2 2 NH3

HNO3

Fig. (3). Proposed decomposition pathways for GN and TAGN (Damse et al. [30]).

Thus, previous research on guanidinium-based compounds with nitrate anion shows some evidence supporting that these com-pounds can act HEDMs. Nitric acid is one of the important products formed when they decompose and it plays in important role in the overall decomposition by reacting further with other product spe-cies. The structure of the nitrate anion is simple as compared to that of heterocyclic-ring containing anions which appear along with guanidinium-based cations in many ionic compounds which are of interest to the high-energy materials research community. Such compounds have been discussed in the next section.

4. DECOMPOSITION OF GUANIDINIUM-BASED COM-POUNDS WITH HETEROCYCLIC-RING ANIONS

Azotetrazolate salts of guanidinium and amino-substituted gua-nidinium-based cations are among the most widely studied ionic compounds in the field of energetic materials in recent times [36, 39, 41, 44, 49, 51]. The stated motivation of the respective research groups in studying them is that both their anions as well as cations have a high-nitrogen content, endowing them with the ability to release enormous amount of energy on decomposition. The mecha-nism of this energy release is different from that observed in con-ventional energy intensive materials and also has some added bene-fits. Energy generation mechanism in composite propellants, which contain little or no nitrogen, is the combustion of a metal fuel such as Al and polymeric binders along with oxidisers, a common one

being ammonium perchlorate. Other materials such as TNT, RDX, and HMX derive their energy from rapid oxidation of the carbon backbone by built-in oxygen. Still others such as CL-20 and the recently reported hepta and octanitrocubanes have additional fea-tures of strained molecular structure. But in high-nitrogen com-pounds, which are the subject of current discussion, the presence of N-N and C-N bonds confers high positive heats of formation, which dissociate during combustion to form the source of their energy. Further, concentration of nitrogen gas is higher in their product gases as compared to most of the other HEDMs, resulting in inher-ently cooler combustion products. This feature is desirable in gun propellants and gas generators. These compounds also have a low proportion of molecular carbon and oxygen, which reduces the proportion of oxidized combustion products in comparison to con-ventional HEDMs, leading to formation of low mean molecular weight combustion products like methane. Many of the high-nitrogen ionic compounds are known to have a high theoretical specific impulse. In general, their thermophysical properties are also appreciable, such as high densities (>1.50 g/cm3), good thermal stabilities, distinctive decomposition temperatures between 140 and 260 °C and friction and impact sensitivities within the generally prescribed limits. All these attributes make them suitable for use as gas generators for airbags, initiators or additives in propellants, constituents of pyrotechnic compositions and low-smoke propellant ingredients. As already mentioned TAGzT which falls in this cate-gory exhibits one of the fastest low-pressure burning rates measured till date for an organic compound [39].

Hiskey et al. [31] synthesized guanidinium azotetrazolate (GzT) and TAGzT and subjected them to nuclear magnetic resonance (NMR) spectroscopy and X-Ray crystallography. They reported some important physical and thermodynamic properties of these nitrogen-rich ionic compounds. Their motivation was to explore whether any of these materials can be used as a direct replacement for HMX in composite propellants as they can give comparable performance along with cooler and less reactive product gases. According to them GzT could also be a replacement for sodium azide in safety equipment because the product gases would be cool and inert. Excellent impact and thermal stability, and absence of problems associated with toxicity and sodium hydroxide production from sodium azide are additional advantages.

In the late 1990s researchers started proposing semi-global or elementary reaction schemes for decomposition of energetic ionic compounds in the condensed phase based on experimental results, rather than just identifying the decomposition products. These ex-periments were either slow decomposition tests such as TGA and DTA or involved fast pyrolysis of compounds. GzT and TAGzT were also the focus of slow decomposition studies (TG-DTA, DSC) carried out by Sivabalan et al. [36]. They found the compounds to be thermally stable up to 180°C. The decomposition products de-tected by them using FTIR spectroscopy were same as the ones later found by Hammerl et al. [38]. Sivabalan et al. also incorpo-rated TAGzT into solid propellant formulations and concluded from DSC that it does not hamper the thermal stability of the double-base matrix consisting TAGzT, dense nitrocellulose and casting liquid. Burn rate measurement done by them on the formulations by using acoustic emission technique demonstrated that TAGzT acts as an efficient energetic additive. The fact that TAGzT enhances the burn rate, when combined with conventional energetic materials such as RDX, was also demonstrated by Kumbhakarna et al. via numerical simulations [42]. Their three-phase model employed detailed chemical kinetics for RDX but for TAGzT they had to rely on


global reactions due to lack of chemical kinetics data in the litera-ture prevailing at that time.

Hammerl et al. [38] characterised the ionic salts: GzT, ammi-noguanidinium azotetrazolate (AGzT), diaminoguanidinium azotetrazolate (DAGzT) and TAGzT using IR and Raman spectros-copy, multinuclear NMR spectroscopy and elemental analysis. In addition they proposed decomposition pathways for the ionic salts from the product compositions obtained by performing DSC stud-ies. The gaseous products were identified by MS and IR spectros-copy. Chemical decomposition mechanisms proposed by Hammerl et al. for GzT, AGzT, DAGzT and TAGzT are summarised in Figs. 4 and 5. According to them the azotetrazolate anion (AzT2�) de-composes via the protonated species i.e. they subscribe to the hy-pothesis of a proton transfer occurring from the cations to the dian-ion as the initiation step. They also support the theory that decom-position of the azobistetrazoles is initiated by ring-opening reac-tions in which the tetrazole ring decomposes either to the corre-sponding nitrile along with the release of hydrogen azide or to the nitrilimines along with the release of elemental nitrogen (pathways 1 and 2 respectively in Fig. 4). According to them pathway 2 seems to dominate due to the absence of hydrogen azide (HN3) in the IR as well as the MS of GzT, AGzT, DAGzT, and TAGzT. As for the guanidinium based cations, Hammer et al. propose that either am-monia (GzT) or hydrazine (AGzT, DAGzT, and TAGzT) is elimi-nated at the beginning, yielding the observed carbodiimide (Fig. 5, reaction 1). The elimination of hydrazine was evident from the observation of m/z=32 (N2H4) in the mass spectra of AGzT, DAGzT, and TAGzT. Hydrazine also partially decomposes quickly according to known mechanisms to form N2, H2, and small amounts of NH3. Consecutively more NH3, HCN, and N2 appear in the gas-phase due to decomposition reactions of carbodiimide (Fig. 5, reac-tion 2).

Most of the studies involving TAGzT decomposition that are reported in literature are conducted using slow heating rates, which are typical in TGA and DSC studies. But Tappan et al. [39] opted for flash pyrolysis/FTIR spectroscopy by which they were able to

expose small TAGzT samples (approximately 200 �g) to heating rates as high as 2000 °C/s. Their analysis resulted in the detection of cyanamide, dicyandiamide ((NH2)2CNCN), NH3, and HCN as the decomposition products. They were not able to quantify cyana-mide and proposed the reaction mechanism shown in Fig. 6 for TAGzT pyrolysis. Reaction 1 shows the global decomposition of TAGzT to the products detected by FTIR spectroscopy. Reaction 2 is a result of calculations done by Tappan et al. considering a rocket chamber at 6.8 MPa and 0.1 MPa exhaust pressure. They assume that condensed-phase products observed in the flash pyrolysis ex-periment react to from final products that appear in their calcula-tions. Their work also includes burn rate measurements and laser ignition of TAGzT. According to them condensed-phase reactions dominate its decomposition and ignition behaviour and are respon-sible for the release of about 65% of energy, which helps explain the high burning rates at low pressure.

The reaction mechanism proposed by Chowdhury and Thynell [46] for decomposition of TAGzT to explain the formation of spe-cies which they observed in their confined rapid thermolysis tests is shown in Fig. 7. The tests were carried out in conjunction with rapidscan FTIR spectroscopy and time-of-flight MS (TOFMS) with heating rates of around 2000 K/s and at temperatures around 260 °C. They also support the conclusion of Tappan et al. [39] that con-densed-phase reactions control the ignition and decomposition be-haviour. According to the decomposition pathways theorised by Chowdhury and Thynell, initiation is expected occur due to proton transfer, which gives rise to the neutral species triaminoguanidine (TAG) and various isomers of azobistetrazole (AzT). Their pro-posed reaction scheme is largely similar to that of Hammerl et al. [38] and contains all the same species with the exception of isocy-anamide (H2NNC). Both these groups did not detect HN3 in their experiments.

GzT and TAGzT were also studied by Damse et al. [30]. Their TG-DTA tests for GzT revealed a two-stage weight loss process, of which the first stage indicates an exothermic rapid reaction in the temperature range of 200-259 °C corresponding to 59% weight

NHR2

R1

+NHR3

N

N

N

N-

N

N

N

N-

N

N

N

N

N

N

N

N

N

H

NN

NH

N

N

N

N

N

N

N

H

NN

NH

N

N

N

N + 2HN3 + N2

+ 2N22 HCN + 2 N2 N

N C+

NC+

NN-

HN-

H

Pathway (1)

Pathway (2)

2

(CN)2

Fig. (4). Proposed decomposition Pathways for the 5,5’-Azotetrazole structure in GzT, AGzT, DAGzT and TAGzT (Hammerl et al. [38]).

NHR2

NHR1

+NHR3

or + HNCNH (1)

1/3 NH3 + HCN + 1/3 N2 (2)

NH3

N2H4

HNCNH

Fig. (5). Proposed decomposition pathway for the Guanidinium cations in GzT, AGzT, DAGzT, and TAGzT (Hammerl et al. [38]).


loss, and the second stage shows slow endothermic reactions above 259 °C. As shown in Fig. 8, Damse et al. explained these two stages in by theorizing that the first stage is the transformation of azotetrazolate anion (AzT2�) to a highly unstable cation (AzT2+) by extraction of acidic protons available with Gu+. This is followed by opening of the azotetrazolate ring with the release of N2 and NH2CN. Damse et al. justified this hypothesis by stating that the theoretical masses of the fractions of N2 and NH2CN within the GzT molecule is approximately equal to the mass loss observed in their TGA experiment. The second stage of slow endothermic reac-tions corresponds to the breakage of relatively strong C-N bonds (770 kJ/mol) as compared to N-N bonds (159 kJ/mol). While for-mulating a similar reaction mechanism for TAGzT based on TGA results, Damse et al. highlighted that unlike GzT, AzT2� in TAGzT cannot remove a proton from the triaminoguanidinium cation (TAG+) because the substituted amino groups present a steric hin-

drance. Therefore, decomposition was initiated instead by the cleavage of the azo group is preferred. The proposed mechanism (Fig. 9) was also supported by the argument that the percentage of mass loss in all the four steps observed for TAGzT in the TGA profile matches with theoretical mass of the fractions of N2, (NH2)6,(N2)3, and HCN in the molecule.

Hayden also tried to decipher the underlying chemistry of GzT and TAGzT decomposition [45]. Her motivation was to explore high-nitrogen burn rate additives to meet growing demands of fu-ture high-performance gun systems. The experimental techniques that were used were simultaneous thermogravimetric modulated beam MS (STMBMS) and Fourier-Transform ion cyclotron reso-nance (FTICR). Hayden, after a series of experiments at various conditions, put forward complex reaction networks to explain her experimental data for these compounds. They are shown in Figs. 10-12 for GzT and in Figs. 13-15 for TAGzT. For GzT, Hayden’s

TAGzT 5.08 NH3 + 4.00 HCN + ? NH2CN + 6.46 N2

5.08 NH3 + 4.00 HCN 4.50 N2 + 5.65 H2 + 2.53 C + 1.47CH4

(1)

(2)

Fig. (6). Proposed global decomposition reactions for TAGzT (Tappan et al. [39]).

NHC+

HN

HN

NH2 NH2

NH2

N

NN

N-

NN

N-

N

NNNH

C+

HN

HN

NH2 NH2

NH2

H

N

NN

N

NN

NN

NN

H

HH

N

NN

N

NN

NN

NNNH

N

HN

NH2 NH2

NH2

NH2 NH

N

HN

NH2

NH2

NH

N

HN

NH2

NH2

N

HN

NH2

NH2

-CN+

NH2

Partial

NH3

ñ N2

-HNN

C+ NN

NN

NN

H

H2N

NN

NN

H

HH

HN

HN

NN

NN

NN

ñ N2

ñ N2

ñ HCN

H

-N

N+

N

NN

NN

ñ N2

H

H2N

NN

NN

+ +

+

+

+

+

+

+ +

+

2

Unstable nitrene

Unstable nitrene

N2H4

N2

HCN

Fig. (7). Proposed decomposition pathways for TAGzT in the condensed phase (Chowdhury and Thynell [46]).


hypothesis from her results is based upon the proton transfer theory. The reaction scheme proposed by her is, in most part, consistent with previous works [38, 47], but with some additional compo-nents. Drawing from her comprehensive set of results from differ-ent types of experiments she accounted for residue formation through mutual reactions between decomposition products of the neutral guanidium (Gu) and AzT molecules. Discussion on the decomposition of this residue is also included in her work. Moreo-ver, unlike Hammerl et al. [38], Hayden identified carbodiimide (CH2N2) as one of the major products and stated that HCN and N2

are primarily formed from the non-residue resulting from GzT de-composition.

The differences between TAGzT decomposition scheme pro-posed by Hayden and those proposed by others previously [30, 36, 38-39] are similar to the differences discussed above for the GzT scheme. Additionally there are reactions involving interactions of the residue products with TAG and AzT which is clear from Fig. 15. Hayden has discussed the various chemical processes occurring in different phases in detail in her work and has concluded on cer-tain aspects of the behaviour of GzT and TAGzT. According to her, GzT does not alter the burn rate of conventional propellants such as RDX when used as a burn rate modifier because it decomposes at a temperature which is above the melting point of RDX and does not generate hydrazine on decomposition. On the other hand TAGT decomposes at a much lower temperature, below the melting point of RDX, and alters the initial stages of the decomposition of RDX. It generates hydrazine on decomposition which reacts faster with RDX compared to the rate of decomposition of RDX itself. Hence TAGzT is a very effective burn rate enhancer.

Interest in GzT and TAGzT in the energetic materials commu-nity has been continuously growing, as evidenced by new research emerging regularly in literature. Very recently, An et al. [41] syn-thesized GzT, characterized it, and formulated a reaction scheme explaining their TGA, DSC, in-situ thermolysis/FTIR spectroscopy and pyrolysis/MS results. The process of initiation of decomposi-tion that they proposed was the same as that postulated by most of the researchers which consists of deprotonation of the cation. They identified a number of species and molecular fragments over a range of molecular weights 20 to 207 through MS and explained the formation of these species hypothesizing a set of chemical reac-tions, shown in Fig. 16. As per their hypothesis, guanidine and azotetrazole decompose independently. Guanidine can ultimately lead to the formation of melamine or its polymer through a pathway that involves breaking of the C-N bond followed by the formation of the isomers cynamide and carbodi-imide. On the other hand

N

N

N

N

N-

N

N

N N

N- NH2

+H2N

NH2

NH2

+H2N

NH2

protontransfer

(4H+)NH2

-N

NH2

NH2

-N

NH2

N

N

N

N

NH2+

N

N

N N

NH2+

1st stagering opening

- 3N2

-2 NH2

NH2

N-

H2N

Breakage ofC-N bond

- 2NH2

2nd stage

C NH2N2

2

GzT


4/3 NH3 + 4 HCN + 4/3 N2

4/3 NH3 + 1/3 N2

CN

4 NH2CN

2 NH2

Fig. (8). Proposed decomposition pathways for GzT (Damse et al. [30]).

N

N

N-

N

N

N

N-

N N

NC NH+

NH

NH

H2N

H2N NH2

C NH+

NH

NH

H2N

H2N NH2

1st stageexo

N=N

NN-

NN

C

+HN

HN NH

NH2 NH2

NH2

NNH

N

C

N

C

N

HN NH

2nd stage

homolysisexo

NH2

2

2

-6

3rd stagering opening

- 3N2

-2HCN

C

N

HN NH2

4th stageEndothermicbreakage of

C-N bond

2 HCN + N2


+ N2NH26

TAGzT

4 NH3

Fig. (9). Proposed decomposition pathways for TAGzT (Damse et al. [30]).


Thermal decomposition scheme of GUzT

NH2

NH2+

H2N N N

N

NN-

N

N-N

NN

NH2

NH2+

H2N NH2

NH

H2NN N

N

NHN

N

NHN

NN2 (g)

+ (c) (R1)

GUzTGuanidine(59.0478)

Azobitetrazolate (ABT)

Initial Steps

Guanidine Related Reactions

NH2

HN

NH2

N

N

NH2N

NH2

NH2

+ (g)

(17.0625)3 (c)

N

N

NH2N

NH2

NH2

(c)

N

N

NH2N

NH2

NH2

(g)

(126.0654)NH2

HN

NH2

(g)

(17.0625)+

C NHHN or C NH2N

(42.0212)

(g)

(R2)

(R3)

(R4)

Guanidine (59.0478)

Guanidine

or

C NHHN or C NH2N

(c)

2

CN (g)(27.0103)

+

N

N

NH2N

NH2

(c) + (g)

(17.0625)

N

N

NH2N

NH2

(c)

N

N

NH2N

NH2

(g)

(111.0545)

(R5)

(R6)

3 NH3

C3H6N6

C3H6N6

NH3

CH2N2

XH

X NH3

C3H5N5

C3H5N5

Fig. (10). Proposed Gu+ related reactions for GzT decomposition (Hayden [45]).

ABT Related Reactions

N N

N

NHN

N

NHN

NN

+

N

N

N

N

N N

or N N

NHN

HN

N

(110.0336)ABT (166.04639 not obs.)

N

N

N

N

N N

N NN

H

NN N

N N

N NN N

H

n - 2

1,2-dimethaniminyldiazene polymer ABT residue

n

n N2+

(R7)

(R8)

Mixed Residue Reaction

N

N

NH2N

NH2

NH2

N

N

NH2N

NH2

F1 F2

N N

N

HN

H2N NH2

HN

N NH

HN N

N NH

HN N

NH

NN

N NH2

N NN N

H

F1 (n-2) F2 (n-2) (1-F1-F2)*(n-2)

(R9)

2 N2

C2H2N6

[H]

Fig. (11). Proposed AzT2� related reactions for GzT decomposition (Hayden [45]).


Residue Decomposition Reactions

N N

N

NH

H2N NH2

HN

N NH

HN N

N NH

HN N

HN

NN

N NH2

N NN N

H

F1 (n-2) F2 (n-2) (1-F1-F2)*(n-2)

Mixed Residue

(27.0103)

+ + + +C NHHN

(28.0056) (17.0625)

N N

NH

N

N

N

NH

N

N

NH2N

NH2

NH2(42.0212)

(166.0715) (126.0654)

N

N

NH2N

NH2

(g)

(111.0545)

+

+

(R10) HCN N2 NH3

CH2N2

C4H6N8 C3H6N6

C3H5N5

Fig. (12). Proposed residue decomposition reactions for GzT (Hayden [45]).

Thermal decomposition scheme of TAGzT

NH

NH+

HN

NH2

NH2 NH2

N N

N

NN-

N

N-N

NN

N N

N

NHN

N

NHN

NN2 (g)

+ (c) (R1)

TAGzT Azobitetrazolate (ABT)

Initial Steps

NH

NH+

HN

NH2

NH2 NH2

NH

N

HN

NH2

NH2 NH2

TAG104.0805

TAG Related Reactions

NH2H2N +Not observed (72.04305)(32.0369)

HN

N

HN

H2N

NH2

NH2

C NNH2N NH2

TAG (104.0805)

C NNH2N NH2 + HCN + N2

C NNH2N NH22 + NH

N

N

HN

NH2

+

(99.0539)

NH

N

N

HN

NH2

(g)NHHN

NHN

NH2

NHN

NHNHN (c)+

NHHN

NHN

NH

NHN

NHN

NH

NHHN

NHN

NH2

(c)

nTAG residue

(R2)

(R3)

(R4)

(R5)

NH3

N2 NH3

C2H5N5

Fig. (13). Proposed TAG+ related reactions in TAGzT decomposition (Hayden [45]).


ABT Related Reactions

N N

N

NHN

N

NHN

NN +

N

N

N

N

N N

or N N

NHN

HN

N

(110.0336)ABT (166.04639)

(R6)

N

N

N

N

N N

2 +NHNH

N N

+ N N2 +

(84.04305)NHNH

N N

nN N

N

HN

N NN N

N NN N

H

n - 21,2-dimethaniminyldiazene polymer ABT residue

(R7)

(R8)

Mixed Residue Reaction

NH

N

N

HN

NH2

N NN

HN

N NH

HN

N

N

NH

NH

HN N

N NN N

H

n - 2

(R9)

2 N2

C2H2N6

N2H4 N2

C2H4N4

Fig. (14). Proposed AzT2� related reactions for TAGzT decomposition (Hayden [45]).

Residue Decomposition Reactions

N NN

HN

N NN N

N NN N

H

n - 21,2-dimethaniminyldiazene polymer

ABT residue

+ + N+ N-

(42.0213)

NH

HN NN

(84.04305)

+

(R10)

N NN

HN

N NH

HN

N

N

NH

NH

HN N

N NN N

H

n - 2 Mixed residue

NHHN

NHN

NH

NHN

NHN

NH

NHHN

NHN

NH2

(c)

nTAG residue

3

+ HCN + NH

N

N

HN

NH2

(99.05395)

+NH

N

N

HN

NH2

NH2

(114.0649)+ N+ N-

(42.0213)

Interaction of products

(g)

NH

N

HN

NH2

NH2 NH2

TAG

N N

N

NHN

N

NHN

NN

Azobitetrazole (ABT)

ABT residue

or

Mixed residue

or

TAG residue

+

(c)

NH

N

HN

NH2

NH2 NH2

N NN

H

NN N

H

HN

N

N

NH

NH

HN N

N NN N

H

n - 2

(residue)

N N

N

NHN

N

NHN

NN

(residue)

(R11)

(R12)

(R13)

(R14)

HCN N2

CH2N2

C2H4N4

NH

C2H5N5

C2H6N6

CH2N2

Fig. (15). Proposed residue decomposition reactions for TAGzT (Hayden [45]).


NH2+

H2N NH2

N-

N

NN

N N

NN

NN-

NH2+

H2N NH2

HN

N

NN

N N

NH

N

NN

proton transfer

NH

H2N NH2

NH

H2N NH2

or

NN

NHN

N N

NN

NNH

NN

NHN

N N C+

N

NH-

- N2

main process

-HNN

C+ N N C+

N

NH-

or

N N

N

NHN

HN

or

NN

NHN

N N

N

NH

HNCNH, HCN and N2N

N

NHN

N N N

-HNN

C+ N N N

- N2

- 2N2

C+ N NH-N

- HN3

- and HCN, HNCNH

- NH3

dimerization

dicyandiamidetrimerization

- NH3

melamine ( C3 )

and isomelamine

polymerize reaction- NH3

melam or melem

-N2

-HCN

NC-N3

N3

+NH3

NH4N3

NH2CN or HNCNH

H6N6

- -

Fig. (16). Proposed GzT decomposition pathways (An et al. [41]).

N

N N

N

N

N

N

N

N

C+N

H

H N H

H

N

HH

C+

N

H

H

N

HH

N

H

HC+N

H

H N H

H

N

HH

C+N

H

H N H

H

N

HH

N

N N

N

N

N

N

N

N

2G + + ZT2-

2.091 b

2.205 b

1.984 b

dissociation

Ea= 28.9 kJ/mol

54.7a;1.788b

442.4a

421.9a

386.7a

439.6a

420.9aa

76.7a ;1.718b

444. 9a

397. 4a

417.5a 456. 3a

GZT(-1035.6043c)

GZTwifc(-1035.5986)

N

N

Fig. (17). Proposed decomposition pathway for GzT: (a) bond energy in kJ/mol, (b) bonding distance in Å, B3LYP/6-31G(d,p) method; (c) self-consistent fieldenergy, in au. (Liu et al. [47])

HCN, N2, NH2CN, and NH4N3 are formed from through tetrazole ring opening.

While most of the work done on TAGzT and GzT is experi-mental, with the advent of faster computers in the first decade of twenty first century quantum mechanics based molecular modelling calculations emerged as the mainstay of simulation of reasonably large molecular systems. As a result, the trend of computationally simulating molecular structures picked up and literature containing such work started appearing. Although a significant number of publications containing molecular level calculations have emerged in a short time, those among them focussing specifically on reaction

mechanism development are relatively few. The work of Liu et al.on GzT is an example of such kind [47]. They used the Gaussian 98 suite of programs employing density functional theory (DFT) [B3LYP/6-31G(d,p)] and ab initio [MP2/6-31(d,p)] and [HF/6-31G(d,p)] methods to model structures of reactants, products, and transition states occurring during the decomposition of the ionic compound. They discuss three aspects of the whole process one by one: (i) GzT cracking, (ii) decomposition of Gu+ and (iii) decompo-sition of Azotetrazolate (AzT2�) dianion. The reaction scheme pro-posed by them is shown in Figs. 17, 18, and 19. Liu et al. were also able to calculate activation energies for each elementary reaction.


The elementary reaction scheme proposed by them consists of the dissociation of GzT by transcending a very low barrier of 28.9 kJ/mol to form a complex which further cracks into two G+ and one AzT2� ions as shown in Fig. 17. In G+ cation cracking, as elaborated in Fig. 18, the preferred pathway is the elimination of H+ by cross-ing a barrier of about 210 kJ/mol followed by NH4

+ elimination which requires an activation energy close to 50 kJ/mol. Finally the intermediate decomposes to CN+ and NH3. Fig. 19 gives the details of the AzT2� anion cracking. The most feasible pathway for this process is consecutive step-by-step release of N2 starting with a single ring opening, having an activation barrier of ~250 kJ/mol. The final step in this sequence is the disintegration of the interme-diate into CN� and N2.

Cheng et al., in their work on GzT [50], justified their choice of using DFT to study GzT decomposition by stating that such calcu-lations can play a crucial role in resolving the details not available from experimental results. Their work consists of geometry optimi-zation, frequency calculation of molecular structures of various stable species, intermediates, and transition states appearing in the decomposition process of GzT. Thermodynamic properties of the species were also determined and subsequently used to compute enthalpy and Gibbs free energy changes in the proposed reactions. The proposed reaction scheme (Fig. 20) consists of initiation of molecular-type cracking patterns by heterocyclic ring opening, sequential cracking of the two five-membered rings of GzT and simultaneous release of N2 molecules. This is followed by proton

C+ N

H2N

H2N

H

H

1

C

NH3+

H2N NH

C NHHN

C NH2N

+C N +C

NH2

H2N NH+

C+H2N NH2

NH2

C NHNH

C NH2N

2

5

8

3

4

6

7

NH4+

NH4+

NH4+

NH4+

,Ea (150.2;126.4)

+

.

-.

,Ea (454.7;485.5)

,Ea (751.9;724.6)

,Ea (212.9a ;210.0b )

,Ea (54.3;48.8)

,Ea (76.7;53.8)

,Ea (49.1;57.2)

Ea (64.1;62.5)

,Ea (165.7;146.0)

,Ea (39.9;40.7)

,Ea (339.0;313.0)

2 NH3

TS67

-H

-H

NH2

TS18

TS15

TS12

TS57

TS68

TS23

TS24,

TS56

TS78

TS34

Fig. (18). Proposed decomposition pathways for Gu+: (a) Calculated results from the B3LYP/6-31G(d,p) method, in kJ/mol; (b) Calculated results from the MP2/6-31G(d,p) method, in kJ/mol. (Liu et al. [47]).

NN

NN

NN

NN

NN

N

NN

NN

N

N

N

N

NN

NN

NN N

N

NN

N

NN

NN

N

NN

NN

NN N

N

NN

N N

N N

N

N

NNN N C- N

NN

N

N N

N N

N

N

N

-C N + - +

Ring-opening

,Ea (375.4;300.4)

,Ea(23.6;38.3)

Ring-opening, -N2

,Ea(248.9;282.2)

Ring-opening, -N2

,Ea(477.1;544.0)

,Ea(278.8;309.2)

N-N cleavage

,Ea(426.1;386.7)

,Ea (151.0;190.1)

10C12,E

a(151.0;190.1)

,Ea (67.0;97.9)

-

NN

+

NN

+

,Ea(15;60.4)

+

9

10

11

13

14

12

,Ea(165.1;205.3)

2 N2

CN 3 N2

TS99A

TS9A10

TS910A

TS910A

TS1010A

TS99B

TS10D12

TS

TS10B11

CN5

N2

TS13A14 ,Ea(15.0;60.4)

TS11A14

9A

10A

9B

TS12A13

Fig. (19). Proposed decomposition pathways for AzT2�: (a) Calculated results from the B3LYP/6-31G(d,p) method calculated results, in kJ/mol; (b) Calculated results from the HF/6-31G(d) method calculated results, in kJ/mol. (Liu et al. [47])


transfer, bond cleavage and atomic rearrangements. Although, 15 reactions were envisioned by Cheng et al., they could identify tran-sition states for only 5 of them. Their results revealed that the en-thalpy change (�H) and Gibbs free-energy change (�G) of the net reaction are �525.1 kJ/mol and �935.6 kJ/mol respectively. Main contribution to the large amount of released energy comes from the disintegration of the AzT2� skeleton (�H=�598.3 kJ/mol). The final products that came out of their calculations are in agreement with most of the experimental results available in literature.

Even more recently i.e. post 2010, researchers have commenced attempts to develop comprehensive reaction mechanisms for ener-getic ionic compounds which are suitable for combustion simula-tions to improve their capability. These mechanisms are envisioned to contain elementary reactions for all the possible pathways under various combustion conditions, along with the corresponding chemical kinetics data. On these lines, Kumbhakarna et al. formu-lated a detailed mechanism for the condensed-phase decomposition of GzT [49]. Their mechanism consists of 25 reactions and they

corroborated their own confined rapid thermolysis (CRT)/FTIR and MS experimental findings with it. For each of the 25 reactions, they carried out molecular structure optimization of the reactants, prod-ucts, and transition state and calculated elementary rate coefficients using the transition state theory. Reaction coordinate calculations were also performed to trace the path of each reaction from the reactants up to the transition state continuing towards the products. Kumbhakarna et al. explained the energetic behaviour of GzT based on thermodynamic considerations in their mechanism which is shown in Table 1. They differ with all of the experimentalists in explaining the initiation of decomposition. According to them de-composition is initiated within the AzT2� dianion and proton trans-fer, which is a very rapid, barrier-less reaction, takes place only after ring opening has occurred. They found that the initial ring-opening reaction is endothermic, and most of the subsequent reac-tions are highly exothermic. The final products that they showed in their reaction mechanism are N2, HCN, and guanidine. As per their simulations the main source of N2 and HCN is AzT2�, released by

NN

N

N

N

N

N

N

N

N

N+

NN

H H

H

H

H

H

N+

N N

HH

H

H

H

H

N+

NN

HH

H

H

H

HN

N

N

N

NN

C+N

-N2

NN

N

N+

N

H

H

H H

H

H

N+

NN

HH

H

H

H

H

N

N

N

N

NN

C+N

-N2N

N+

N

H

H

H H

H

H

+ N N

-N

N+ C-

N+

N

C+

H

N

N2-

N N

NH H

H H

H

N

N+

N

H

H

HH

H

H

NN

N N

NH H

H H

HN+

N N

HH

H

H

H

H

N N +

+ N

N

N

H

H

H

H

H

N

N

N

H

H

H

H

H

+N

N

N

H

H

H

H

H

NC

H H

H

H

HN

NN

N

-C

N+

H

C-

N+

H

N

NN N2+C-N+H2

C NH2

H H + C NH N

H

H H+ H H

N N

NH H

H

N NCN

H

H

H

H

H+

path 15

path 13

path 9

path 1 path 2

- N2 path 3

path 4

- N2

path 5

path 6

path 7

path 8path 10

path 14

path 11

path 12

2

-N

N+ C-

N+

N

C+

H

N

N2-

N+

N N

HH

H

H

H

H

-N

N+ C-

N+

N

C+

H

N

N2-

-N

N+ C-

N+

N+

C+

H

N+

N-

H

-N

N+ C-

N+

N+

C+

H

N+

N-

H

M0M1

M1

M2M2

N2

N2

M22

M21

M22

M22sp

M221

M211sp

M21sp

M221

N2

N2

NH3

CH2NH

H

HNC

HCN

H2 HCN

N2

Fig. (20). Proposed decomposition pathways for GzT (Cheng et al. [50]).


Table 1. Proposed decomposition pathways for GzT (Kumbhakarna et al. [49]).

No. Reaction �HR(kJ/mol)

R1) N

NN

N-

NN

N-N

NN

-N

C-

N+N

NN-

N

NN +INT4

TS4N2 84.5

R2) N

NN

N-

NN

N-N

NN N

N

NN-

N

NN +TS9

N3-

INT9

154.8

R3) -N

C-

N+N

NN-

N

NN +NH2C+

NH2

H2N

Gu+

+NH2

NH

H2N

Guanidine

-N

N+ N

NN-

N

NN

INT4aINT4

�96.2

R4)

TS4a1+

-N

N+ N

NN-

N

NN

INT4a

N+

N- N-

N

NN

INT4a1

N2 �104.1

R5)

TS4a1f

NH2C+

NH2

H2N+

N+

N-

N+

N-

INT4a1d

+ +NH2

NH

H2N

GuanidineGu+

INT4a1

N+

N- N-

N

NNN2 32.6

R6)

TS4a1i

NH2C+

NH2

H2N+

INT4a1i

+

Gu+

INT4a1

N+

N- N-

N

NN NH2

NH

H2N

Guanidine

N+

NN-

N

NN 73.2

R7)

NN+

C-

isocyanoethene

N+

NN-

N

NN

INT4a1i

TS4a1i1

+ 2N2�322.1

R8)

TS4a1h

NH2

C+

NH2

H2N+

INT4a1h

+ +

Gu+

INT4a1

N+

N- N-

N

NN

H2N

NH2

NH2

N+ C- 2N2HCN�351.8

R9)

TS4a1j

NH2

C+

NH2

H2N+ +

Gu +INT4a1

HCN+

N- N-

N

NN NH2

NH

H2N

Guanidine

+ + 2N2HCNHNC�361.9


Table 1. contd…


R10)

HCN+

N-

N+

N-

INT4a1d

+TS4a1d1

2HCN N2�455.2

R11) +

TS4a1h1-

H2N

NH2

NH2

N+ C-

INT4a1h

NH2

C+

NH2

H2N

Gu +

CN�54.3

R12) NH2

NH

H2N

Guanidine

H2CN

N+

C-

isocyanoethene

+ + -

NH2

C+

NH2

H2N

Gu +

+TS8CN HCN

�157.3

R13) + -

NH2

C+

NH2

H2N

Gu +

NH2

NH

H2N

Guanidine

+CNTS7

HCN �16.3

R14) NH2

NH

H2N

Guanidine

NH2

C+

NH2

H2N

Gu +

+ H2NNH2

H2N

NH2

C+NH2

HNTS1

INT1

�5.8

R15) NH2

C+H2N

NH

NH2HN

INT1d1

H2NNH2

H2N

NH2

C+NH2

HN TS1d1

+

INT1

NH3 10.8

R16) NH2

C+H2N

NH

NH2HN

INT1d1

TS1d1a+

NH2

NH

H2N

Guanidine

C+H2N

NH2

NHHN

NH2

NH2N

INT1d1a

+ NH3 �9.6

R17) NH2

C+H2N

NH

NH2

HN

INT1d1

TS1d1c+

NH2

NH

H2N

Guanidine

+NH

HN

H2N

NH

H2N

C+

NH

H2N

INT1d1c

NH3 47.6

R18)

Guanidine

+NH2

NH

H2N

TS5NH3HNCNH 79.0


Table 1. contd…


R19)

C+H2N

NH2

NHHN

NH2

NH2N

INT1d1a

C+H2N

NH

NH2HN

NH2

N

H2N

INT1d1a1a

TS1d1a1a�18.4

R20)

C+H2N

NH

NH2

HN

NH2

N

H2N

INT1d1a1a

TS1d1a1a1

H2N

NH

C+NH2

HN

NH2

N

H2N

INT1d1a1a1

2.1

R21)

TS1d1a1a1a

H2N

NH

C+NH2

HN

NH2

N

H2N

INT1d1a1a1

H2N

N

C+NH2

HN

N

NH2

Melamine +

+ NH3�66.9

R22)

TS1d1c1

NH

HN

NH2

NH

H2N

C+

NH

H2N

INT1d1c

NH

NH

C+

NH2

NH

H2N

HN

H2N

INT1d1c1

�27.6

R23) NH

HNC+

NH2

NH

H2NNHH2N

INT1d1c1

TS1d1c1a

H2N

NHC+

NH2

N

NH

NH

INT1d1c1a

+ NH3�43.0

R24) H2N

NHC+

NH2

N

NH

NH

INT1d1c1a

TS1d1c1a1

Melamine +

H2N

NC+

NH2

N

NH

NH2

�46.0

R25)

TS4a1d+HC

N

NN-

N

NN

INT4a1 Melamine +

H2N

N

NH2

N

N NH2

Melamine

HCN+

N-

N+

N-

INT4a1d

+ +H2N

NC+

NH2

N

NH

NH2

N2�33.4

the disintegration of the two tetrazole rings in this dianion. But most of the proposed reactions are bimolecular and involve Gu+

also. Particularly the reactions in which unstable intermediates come apart to give stable products were found to be highly exo-thermic and were stated to be the main cause of energetic nature of GzT. The major pathway for NH3 and melamine formation is the reaction between Gu+ and guanidine followed by many subsequent steps.

Going by the current trend in the area of energetic materials, as far as ionic salts are concerned, azobistetrazolate salts appear to be the most promising. However compounds such as guanidinium 5-aminotetrazolate (GA), which have only one tetrazole ring as op-posed to two in azobistetrazolates cannot be overlooked. Experi-ments have showed that GA is thermally quite stable and insensi-tive to friction and impact [52]. It melts at �397 K, and its thermal decomposition has an onset temperature of 167 °C. One of the


compounds considered by Damse et al. [30] in their work on nitro-gen-rich gun propellants is GA. Their TGA-DTA results for GA show an endothermic melting at 124°C, followed by a four-stage weight-loss process, first three stages of which are weakly exo-thermic and the fourth one is endothermic. The products identified from TGA-FTIR and GC-MS were HN3, NH3, and NH2CN as the decomposition gases evolved in the first, second, and third stages of the decomposition process, respectively. Melamine was also found as an intermediate species in the gas phase. As per Damse et al. the release of HN3 in the first stage denotes the ring opening of hetero-cyclic tetrazolate, which leads to the formation of residue compo-nents, guanidine and cyanamide. They propose that simultaneous reactions must be taking place among the residue components form-ing the thermally stable intermediate species melamine.

Using quantum mechanics based ab initio calculations, Kumb-hakarna and Thynell developed an extensive reaction mechanism for GA decomposition consisting of 55 species and 85 elementary reactions [48]. These reactions include unimolecular decomposi-tion, bimolecular and ion recombination, as well as proton transfers and isomeric rearrangements. They could not identify a transition state for proton transfer within the liquid phase from Gu+ to the aminotetrazolate anion (AmTz�) in the GA molecule. They pro-

posed that reactions are initiated between the ion pair Gu+ and AmTz� to proceed through multiple pathways involving various intermediates and initiation via direct ring opening to release N2 is not an important pathway. A wide variety of reaction pathways was investigated in detail. From the 85 reactions, Kumbhakarna and Thynell identified the critical ones by carrying out a sensitivity analysis, given in Table 2. They could not identify a proton transfer reaction at the beginning in spite of an extensive search for the corresponding transition state. Decomposition of GA is initiated by formation of a complex due to the proximity of Gu+ and AmTz�

with the simultaneous release of NH3. This is followed by ring opening which results in the release of N2. The overall picture pre-sented by Kumbhakarna and Thynell is that decomposition at first proceeds through endothermic reactions, but is later replaced by exothermic reactions producing the low molecular weight gases N2,NH3, and HN3. They also used their mechanism to simulate TGA and DSC experiments existing in literature [52] and were able to achieve a good match with the experimental data.

From the data compiled on guanidinium-based compounds, broadly it may be observed that most experimental studies propose that deprotonation of the cation is the initiation reaction during the decomposition of these compounds, based on the observed species.

Table 2. Proposed decomposition pathways for GA (Kumbhakarna et al. [49]).

No. Reaction �HR (kJ/mol)

R1)

NH2

N

NN

N

NH2

H2N

H2N

NH

N

NN

N

H2N

H2N

INT5a1

+ NH3

TS5c

INT5

46.0

R2)

NH

N

NN

N

H2N

H2N

INT5a1

NH

N

N

N

NH2

H2N

NINT5a1b

TS5a1b

�8.7

R3) NH2

H2N

H2N

NH2

NN

N

N

INT6c

H2NNH2

H2NNH2N N

+TS6c

INT6

N2 74.0

R4)

NH2

N-

NN

NC+

NH2

NH2

H2N+

Gu + ATz-

NH2

NH

H2N+ +NN

H2N

Guanidine

N2

TS13

23.4

R5) NH2

NH

H2N+ N

H2N

HN

N

NH

NH2

INT7a1aCyanamideGuanidine

+ NH3

TS7a1a

17.5

R6)

N

N

NH2

H2N

INT5a1b1

+

N

N

NH2

H2N

INT5a1b1

NH

N

NH2

H2N

N

N

NH2

NH

INT69a

TS69

�18.8

R7)

NH2

N-

NN

NC+

NH2

NH2

H2N+

Gu + ATz-

C

HN

C

NN

N

N

NH2

NH2NH3

INT7

TS7

14.6


However, the neutral species formed by deprotonation, if indeed they are formed, are not typically detected in the gas phase, possi-bly owing to their reactivity. Hence, the experiments inherently are unable to provide a concrete explanation behind such a hypothesis. Contrarily, computational results obtained by various researchers do not show evidence of initiation by proton transfer. According to these results, deprotonation does occur after the initiation step, which is typically the independent scission of bonds within the cation and the anion to form reactive species. Computations and experiments broadly seem to agree that the main source of energy release in all guanidinium-based compounds with nitrogen-rich heterocylic anions are the reactions leading to ring-splitting.

5. DECOMPOSITION OF IMIDAZOLIUM-BASED IONIC COMPOUNDS WITH HALIDE ANIONS

The decomposition pathways of imidazolium-based ionic com-pounds with halide anions were studied extensively in the literature using both experimental and numerical techniques. Typically chlo-ride and the bromide anions were chosen to provide further insights into the initiation pathways without complicated secondary reac-tions involved with the oxygen-rich energetic nitrate anion.

In one of the earliest works on thermal decomposition of ionic halides, pyrolysis of a wide range of 1,3-disubstituted imidazolium salts, R1R2Imidazolium X (R1 , R2 = methyl, ethyl, propyl, isopro-pyl, butyl, benzyl, vinyl, phenyl, and allyl; X = Cl� , Br� , I�) was conducted by Chan et al. [53]. Quantitative analysis of the products was carried out by NMR spectroscopy and gas chromatography. The compounds were subjected to temperatures ranging from 220 to 260 °C for 0.5-1.5 h under vacuum, and were found to form 1-substituted imidazoles. 1-ethylimidazole was found to dominate the products over 1-methylimidazole, and no traces of HX were de-tected. The predominance of 1-ethylimidazole during decomposi-tion was attributed to an SN2 attack of the highly nucleophilic anion on the methyl group.

Confined rapid thermolysis, aided by FTIR spectroscopy and TOFMS, of two imidazolium salts, with 1-ethyl-3-methylimida-zolium as the cation, and Cl� and Br� as the anions, was conducted

by Chowdhury and Thynell [54]. The rapid pyrolysis, conducted at heating rates of 2000 K/s, of the chloride and the bromide com-pounds were found to be similar in nature, with reactions being initiated beyond 390 °C, while the gaseous species from the con-densed phase reactions being detected around 420 °C. The FTIR spectra and the mass spectra were compared with spectra of possi-ble by-products to determine the presence of n-methylimidazole, n-ethylimidazole, ethyl chloride or bromide, and methyl chloride or bromide. The abundance of n-ethylimidazole over n-methylimida-zole demonstrated that the dominant decomposition pathway was the transfer of the methyl group from the cation, while the secon-dary pathway involving the transfer of the ethyl group was also active, as depicted in Fig. 21.

Quantum chemical calculations using the B3LYP level of den-sity functional theory and the 6-31G** basis set were used to con-struct a predictive tool to determine the decomposition pathways and the associated kinetic parameters by Kroon et al. [55]. In this study as well, the highly nucleophilic halide anions were found to influence the decomposition of the ionic liquids by the dealkylation of the cation. As shown in Fig. 22, the decomposition of 1-butyl-3-methylimidazolium chloride (BmimCl) proceeded through an SN2type of attack on the sterically more accessible alkyl group, i.e. the methyl group, to form 1-butylimidazole and mehyl chloride. The activation energy of the process was also calculated and was found to be 127 kJ/mol, which was lower than the activation energy of the secondary pathway of formation of 1-methylimidazole and butyl chloride by 9 kJ/mol. It was also predicted that the reaction was overall endothermic. The effect of the length of the alkyl chain on the decomposition process was explored for various 1-alkyl-3-methylimidazolium chlorides, where the alkyl group was ethyl, propyl, butyl, hexyl, and octyl. As seen in Table 3, the results indi-cate that the decomposition onset temperatures and the associated activation energies were quite invariant with the length of the chain, and the formation of methyl chloride was favoured over the other alkyl chloride in all the cases. The activation energy of the primary pathway was always lower than that of the secondary pathway by 10 kJ/mol. This is expected as the steric hindrance to the SN2 type of reactions would increase with the increase of the alkyl chain length, and a reversal of the primary and secondary pathways would be improbable.

N N+

Cl-

N N +SN2

CH3Cl

Fig. (22). Thermal decomposition of BmimCl into methyl chloride and 1-butylimidazole (Kroon et al. [55]).

Table 3. Calculated activation energy barriers [55] and experimen-tally determined thermal decomposition temperatures (from literature) for thermal degradation of several 1-alkyl-3-methylimidazolium ionic liquids with different al-kyl chain lengths.

Ionic liquid �Ea (kJ/mol) Tdecomp (°C)

EmimCl 126 261

PmimCl 125 261

BmimCl 127 254

HmimCl 128 253

OmimCl 128 254

N

N

C2H5

CH3

-(primary)

+N

N

C2H5

+N

N

CH3

(secondary)

N

N

C2H5

CH3

-(primary)

+N

N

C2H5

+N

N

CH3

(secondary)

Cl

CH3Cl

C2H5Cl

Br

CH3Br

C2H5Br

Fig. (21). Proposed decomposition pathways for 1-ethyl-3-methylimidazolium chloride (EmimCl) and bromide (EmimBr) (Chowdhury and Thynell [54]).


The gas phase as well as the condensed phase decomposition of EmimBr was probed by Chambreau et al. [56] by an array of ex-perimental techniques, namely DSC, TGA-MS, and vacuum ultra-violet photoionization TOFMS (VUV-PI-TOFMS), as well as ab initio calculations using the density functional theory utilising B3LYP and M06 functionals, MP2 and CCSD(T) levels of theory along with the 6-31+G(d,p) basis set. Additionally, molecular dy-namics simulations were conducted using the force field AP-PLE&P, equipped to handle the bromide anion. The TGA-MS re-sults corroborated the findings from the previous studies, and showed the formation of methyl bromide, and ethyl bromide with their activation enthalpies being 116.1 kJ/mol and 122.9 kJ/mol, respectively. The difference between the two activation enthapies was similar to the one determined by Kroon et al. [55]. The values were derived from the TGA-MS curves by an Arrhenius plot de-rived from the rate of product formation plotted against the inverse of temperature. The VUV-PI-TOFMS tests on the compound, con-ducted at temperatures much below the onset temperatures indi-cated by the DSC results, indicated that vaporisation took place through the evolution of methyl bromide, ethyl bromide, ethylimi-dazole, and methylimidazole.

The branching ratio of methyl bromide to ethyl bromide, indica-tive of the prevalence of the primary pathway over the secondary pathway, was found to be 0.76:0.24, which was consistent with the TGA-MS results. The calculated reaction enthalpies for SN2 reac-tions were also found to be in agreement with the experimentally calculated values. As mentioned by Kroon, the increment in the activation energy of the secondary pathway was explained by the presence of steric factors. Finally, the enthalpy of vaporisation and the liquid-phase heat of formation of the compound were calculated to be 168 kJ/mol and �130 kJ/mol at 298 K. The liquid-phase heat of formation 1-butyl-3-methylimidazolium bromide was �180kJ/mol.

Ohtani et al. [57] analysed the thermal decomposition pathways of EmimCl, EmimBr, BmimCl, and 1-hexyl-3-methylimidazolium chloride (HmimCl). The experimental study utilized a vertical mi-crofurnace-type pyrolyzer, coupled to a GC, which was equipped in turn with a flame ionization detector, a non-radioactive-type elec-tron capture detector, a nitrogen-phosphorus detector, and a mass

spectrometer. The separation of products was achieved by a fused silica capillary column and a metal capillary column. The decom-position studies were conducted isothermally at 550 °C, and the pathways determined for EmimBr and BmimCl, shown in Fig. 23,were identical to the ones determined by the previous researchers, with the halide anion preferentially abstracting the methyl group through an SN2 type of reaction as the primary pathway, and the ethyl group as the secondary pathway. The peak heights of the imi-dazoles in the pyrograms bore testimony to the precedence of the methyl abstraction pathway over the ethyl-abstraction pathway.

However, owing to the higher temperatures and the high heat-ing rates applied in this study, alkenes such as ethylene in case of EmimBr, and 1-butene in case of BmimCl were detected in the pyrograms. The alkenes were hypothesized to be formed due to a tertiary pathway, owing to their small signals, along with the forma-tion of the corresponding hydrohalide and the alkylated imidazole through a C�N bond cleavage, although HBr and HCl were not detected among the products. Of course, it is debatable if the terti-ary pathway would instead lead to the formation of the hydrohalide through a proton transfer from the larger alkyl chain, leading to the alkylated imidazole and the alkene. Even at these high tempera-tures, the imidazole ring was found to be intact.

While most of the studies discussed so far were conducted at heating rates of the order of 1000 K/s or below, Dessiaterik et al.[58] reported the study of thermal decomposition of 1,3-disubstituted imidazolium salts, R1R2Imidazolium chlorides (R1 = methyl, R2 = methyl, ethyl, butyl, and hexyl) using IR laser abla-tion. The ionic species generated by ablation were detected through TOFMS using pulsed extraction from the ionisation zone, while the neutral species were detected using vacuum UV photoionisation at 10.5 eV. The temperatures within the laser plume were expected to be approximately 475 K, and the heating rates during the process were estimated to be 108 to 1013 K/s. Based on mass balance and ion signals, approximately 99% of the 100 mg sample was removed as intact salt particles or clusters from the reaction zone, while 1% of the sample decomposed to form the corresponding R1R2Imidaozle and HCl. As shown in Fig. 24, the stable R1R2Imidaozle was formed by migration of the R2 group from the nitrogen to the carbon atom. Hence a new pathway was found to be

N N

H2C CH2 Br NN

1 3a

Br CH3 NN

2b

-

N N

Cl NN

5 6 a

Cl CH3 NN

-

Br

Cl

Fig. (23). Thermal decomposition pathways of 1-ethyl-3-methylimidazolium bromide and 1-butyl-3-methylimidazolium chloride (Ohtani et al. [57]).


initiated at high heating rates, precluding the formation of com-pounds such as R1Cl, R2Cl, and the corresponding imidazoles.

N NR1 R2

+- laser N N

R1

R2

+liquid gas

ClCO2

HCl

Fig. (24). Proposed thermal decomposition pathway of imidazolium salts at high heating rates (Dessiaterik et al. [58]).

6. DECOMPOSITION OF IMIDAZOLIUM-BASED IONIC COMPOUNDS WITH ENERGETIC ANIONS

Only few studies were found in the literature involving the py-rolysis pathways of imidazolium-based salts with anions that ren-dered the resulting compound energetic with possible utilisation as a monopropellant or as an explosive. Two such anions reported were nitrate and N,N’-dinitrourea. Additionally, the recent discov-ery of the hypergolicity of cyanamide-based ionic compounds with nitric acid placed a renewed importance on understanding the de-composition of the dicyanamide-based (DCA) ionic compounds, since they would be subjected to high heating rates owing to the exothermicity of the hypergolic reactions.

Confined rapid thermolysis of Emim nitrate was conducted by Chowdhury and Thynell [54] to elucidate the primary reaction pathways associated with the decomposition of imidazolium-based energetic ionic compounds. The products identified during the de-composition of the nitrate salt at 435°C were H2O, CO2, CO, NO, N2O, CH3OH, acrolein (H2CCHCHO), propionaldehyde (H3CCH2CHO), n-methylimidazole, n-ethylimidazole, methoxy-ethyl-imidazole, and ethoxy-methyl-imidazole, among others. Based on the product distribution, the decomposition was expected to be initiated in a fashion similar to the chloride and bromide, with the primary pathway being the formation of methyl nitrate and n-ethylimidazole, as shown in Fig. 25. The subsequent reactions be-tween the two lead to the formation of methoxy-ethyl-imidazole, and several byproducts during its oxidation by HONO. The decom-position of methyl nitrate also produces several products through well-known pathways. The secondary pathway was also found to be active with the formation of ethoxy-methyl-imidazole and similar smaller products as produced in the primary pathway. Deprotona-tion to form the corresponding acid and the neutral imidazole was not found to be an active pathway for any of the compounds. The preference of the anion for abstracting the methyl group over the ethyl group was attributed to the physical affinity of the anions to the methyl group.

The study by Dessiaterik et al. [58] in the previous section also probed R1R2Imidazolium nitrates (R1 = methyl, R2 = methyl, ethyl, butyl, and hexyl) using IR laser ablation. The decomposition was found to proceed in a fashion similar to the chloride salts, with the formation of R1R2Imidazole and nitric acid. Secondary reactions between the acid and the imidazole were not reported, probably owing to the low reactions temperatures (475 K).

The thermal decomposition of 1-H-imidazolium dinitrourea was studied by non-isothermal TGA-DTA and in-situ FTIR spectros-copy in a heated cylindrical gas cell at a heating rate of 5 K/min by Liu et al. [59]. The TGA-DTA studies revealed that the activation energy of decomposition calculated using the Kissinger method and the Ozawa method were similar and the average was 160.7 kJ/mol. The in-situ FTIR spectroscopy studies in the condensed phase showed the formation of N=C=O groups as the compound was

heated from 20 to 200 °C. The detected gas phase species were N2O, HNCO, NO, and NO2. Although bond dissociation studies are not absolute indicators of decomposition pathways, the bond disso-ciation energies were calculated using the density functional theory utilising B3LYP functionals with the 6-31+G(d,p) basis set. The results from the experimental and numerical studies were combined to determine the decomposition pathways of 1-H-imidazolium dini-trourea, as shown in Fig. 26. The initiation reaction was proposed to be a proton transfer simultaneous with an N-N bond scission in the anion, to form 1-H-imidazole, and two intermediates. The interme-diate CN3H2O3 was expected to form HNCO, N2O, and the OH

N

N

C2H5

CH3

- +N

N

C2H5

+

+ +

+N

N

C2H5

N

N

C2H5

+

N

N

C2H5

OCH3

N

N

C2H5

+

byproducts

+N

N OCH3

C2H5

byproducts

Scheme 1: Primary pathway

N

N

CH3

C2H5

-N

N

CH3

+

+

+ +

+N

N

CH3

N

N

CH3

+

N

N

CH3

OC2H5

N

N

CH3

+

Scheme 2: Secondary pathway

NO3 CH3ONO2

CH3ONO2 CH3O NO2

CH3O NO2 CH2O HONO

NO2 HONO

CH3O

CH3ONO2

HONO

NO3 C2H5ONO2

C2H5ONO2 C2H5O NO2

C2H5O NO2 CH3CHO HONO

NO2 HONO

C2H5O

Fig. (25). Schemes 1 and 2, Proposed decomposition pathways of 1-ethyl-3-methyl-imidazolium nitrate (Chowdhury and Thynell [54]).


radical, which formed HNO3 with NO2. HNO3 was expected to decompose further to form various smaller molecular weight gases.

The first work on imidazolium dicyanamide-based ionic liquids was reported by Kroon et al. [55] using the methodology reported in the previous section. The decomposition pathway of BmimDCA, as shown in Fig. 27, was found to proceed through an SN2 reaction as well, with the formation of primarily methyl DCA and butylimi-dazole, and smaller quantities of butyl DCA and methylimidazole. The activation energy associated with the primary pathway was found to be 160 kJ/mol.

The thermal stability of various ionic compounds with cyano-functionalized anions was studied by Chambreau et al. [60], out of which EmimDCA, BmimDCA, and 1-ethyl-2,3-dimethylimida-zolium dicyanamide (EmmimDCA) were of interest. The experi-mental techniques applied were isothermal TGA to determine the enthalpy of vaporization, non-isothermal TGA to determine the overall activation energy of thermal decomposition, and TGA-MS, GC-MS, and T-jump FTIR spectroscopy to determine the gaseous products of decomposition. VUV-PI-TOFMS was also utilized to study the evolved gaseous species from heated samples in vacuum. The reaction pathways and photoionization potentials were corrobo-rated by a hybrid DFT theory applied at the M06/6-31+G(d,p) level of theory. For EmimDCA, EmmimDCA, and BmimDCA, the heats of vaporization were found to be 140.9, 200.3, and 146.9 kJ/mol at 298 K, while the overall enthalpy of activation was found to be 151.7, 268.4, and 159.5 kJ/mol at 566 K, 618 K, and 560 K respec-tively. The experimentally determined enthalpy of activation was found to be identical to the one determined by Kroon et al. [55] (160 kJ/mol) for BmimDCA.

The proximity of the enthalpies of vaporization and the activa-tion enthalpies of these compounds indicated that thermal decom-position and vaporization would be competing pathways below 573 K. The TGA-MS studies revealed that the major species during the decomposition of EmimDCA were ethylimidazole and methylimi-dazole, with associated activation energies of 160 and 175 kJ/mol, respectively, thus corroborating the previously established alkyl abstraction pathways through SN2 type of reactions. Similarly, EmmimDCA was found to decompose to form 1-ethyl-2-methylimidazole and 1,2-dimethylimidazole. For BmimDCA, al-though a small signal corresponding to butylimidazole was de-

tected, the second largest signal was from m/z = 97, which could not be explained. These pathways were confirmed by the T-jump FTIR studies. The VUV-PI-TOFMS studies conducted at approxi-mately 473 K lead to the detection of two new pathways of decom-position for EmimDCA and BmimDCA. The first one was the for-mation of a neutral carbene and the protonated anion by transfer of a proton from the C2 atom on the cation. The second pathway in-volves the carbene being further attacked at the C2 atom by the acid to eliminate HCN and effectively replace the proton at C2 by NCN. This reaction mechanism is shown in Fig. 28. Although not men-tioned explicitly, the carbene formation would be the first step in the formation of the R1R2Imidazoles from the imidazolium com-pounds studied by Dessiaterik et al. [58].

7. DECOMPOSITION OF TRIAZOLIUM-BASED IONIC COMPOUNDS WITH HALIDE ANIONS

As was the case with the energetic imidazolium salts, thermoly-sis of salts formed by halide anions combined with triazolium cations were studied first to elucidate the decomposition mecha-nism of the salts with oxygen-enriched anions. Unlike the imida-zolium halides, studies on triazolium halides were found to be fewer in number in the literature.

Confined rapid thermolysis, combined with FTIR spectroscopy and TOFMS, of 4-amino-1,2,4-triazolium chloride (4ATCl) was conducted by Chowdhury and Thynell [61]. The products identified during the decomposition of the salt at temperatures of 340 °C in an inert atmosphere were HCl, HCN, NH3, NH4Cl, and 1-H-triazole (1TA). In absence of alkyl groups present as ring-substituents on the imidazolium salts, it was expected that the initiation of decom-position would be through the amino group or through a proton transfer. The initial dominance of HCl over 1TA in the FTIR spec-tra indicated that the primary pathway was the formation of 4-aminotriazole (4AT) and HCl, as seen in Fig. 29. Since 4AT was not directly detected among the products, a separate thermolysis study on 4AT revealed the formation of 1TA, HCN, NH3, and N2

under similar temperatures. The NH3 formed though the decompo-sition of 4AT recombined with HCl to form NH4Cl. A secondary pathway, forming HCl and an imino radical, or NH2Cl through an N-N bond scission, was also envisioned and is shown in Fig. 30.

N+

N

H

H

N+

-O N- N

N+

O-

O O O

H

N+

N- N

O O

HHO

+ . + NN

H

N+

N- N

O O

HHO + N2O +

.

. +

. + NO2 + H2O + O2

NO2

HNCO OH

NO2 OH HNO3 NO

Fig. (26). Proposed decomposition pathways of 1-H-imidazolium dinitrourea (Liu et al. [59]).

N N+N N- N

N N+

H3C

N N N

SN2

Fig. (27). Thermal decomposition of Bmim dicyanamide into 1-butylimidazole and methylated dicyanamide (Kroon et al. [55]).


The secondary pathway was not considered dominant owing to the lack of a path to form HCN, which was a prominent product.

The same method was applied to study the methylated version of 4ATCl, albeit with an iodide anion, i.e. 1-methyl-4-amino-1,2,4-triazolium iodide (Me4ATI) [62]. Additionally, the effect of a rear-rangement of the cation was analysed by thermolysis of the com-pound 1-amino-3-methyl-1,2,3-triazolium iodide (Me1ATI). The decomposition of Me4ATI was reported at 270 °C, with the promi-nent products being NH3, 1-methyltriazole (1MeTA), as well as nitrogen and methyl-iodo-triazole. The lack of methyl iodide pre-vented the postulation of a dealkylation type of reaction pathway similar to the imidazolium halides, and instead indicated the forma-tion of 1 MeTA and NH2I, similar to the secondary pathway of 4ATCl, as shown in Fig. 31. The reactive NH2I was expected to form NH3 and methyl-iodo-triazole by a proton abstraction from 1MeTA, as well as to decompose to form the smaller molecular weight products. The thermal decomposition of the isomer of Me4ATI, Me1ATI, was reported at 290 °C, with the formation of NH3, 3-methyltriazole (3MeTA), N2, and methyl-iodo-triazole. In contrast to Me4ATI, CH3I was detected as a product, which estab-lished the presence of a secondary pathway through N-C bond scis-sion and an SN2 type of nucleophilic attack. The primary pathway and the subsequent reactions were similar to those of Me1ATI,

shown in Fig. 32.

HC

N+

R

N

X~

NC

N

R

HNC

X~

X = S, NCN

N N+

R

C+

X

-HN

N N+

R

X-HN

N N+

R

HN X~

N NR

X~C+

HN

C

HN

+ N NR

X

R= Et, Bu

C

HNCH

N

Fig. (28). Generalized scheme for the addition-elimination reaction through a carbene intermediate (Chambreau et al. [60]).

N

NNH

H2N

H

H

+Cl- HClN

NN

H

HH2N

NH3 +

N

NN

H

H

H

+ HCN + N2

HCl + NH3 NH4Cl

(according to pathways I & II for decomposition of 4AT)

N

NN

H

HH2N

Initiation

Decomposition of 4AT

Fig. (29). Proposed primary reaction pathway for 4ATCl (Chowdhury and Thynell [61]).

+HCl

N

NNH

H

H

HCl + NH3 NH4Cl

NH +N

NN

H

H2N

H

H

Cl-

NH + HNNHNH

NH + NNH + NH2HNNH

NH + N2 + NH2NNH

NH2 + NH2 NH + NH3

NH2 N2 NH3NNH ++

+HCl NH NH2Cl

NH2 NH3Cl++ NH2Cl NH

NH2 NH4Cl++ NH3Cl NH

Initiation

Propagation

Termination

Fig. (30). Proposed secondary reaction pathway for 4ATCl (Chowdhury and Thynell [61]).


Thermal decomposition of 4ATCl was studied by Li and Litz-inger [63] heated by a CO2 laser with a heat flux of 100 W/m2. The gases evolving from the condensed phase were sampled by a triple quadrupole molecular beam mass spectrometer. Under such high heat fluxes, it would be expected that the sample would experience high heating rates. The species detected were HCl, 4AT, HCN, NH3, and 1TA. The presence of 4AT among the products led to the confirmation of deprotonation at the N1 atom as the primary path-way of decomposition, thus corroborating the pathway determined at lower heating rates.

8. DECOMPOSITION OF TRIAZOLIUM-BASED IONIC COMPOUNDS WITH ENERGETIC ANIONS

The thermal decomposition studies of the energetic salts of tria-zolium-based cations were primarily confined to the nitrate salts of the same cations reported in the previous section, namely 4-amino-1,2,4-triazolium nitrate (4ATN), 1-methyl-4-amino-1,2,4-triazolium nitrate (Me4ATN), and 1-amino-3-methyl-1,2,3-triazolium nitrate (Me1ATN). Ab initio calculations were pursued to identify the deprotonation reactions for 1,2,4-triazolium dinitramide and 4-amino-1,2,4-triazolium dinitramide. These studies aside, several TGA and DSC-based studies were conducted to analyse the activa-tion energies and pre-exponential factors related to the decomposi-tion of several nitrate and dinitramide-based salts, such as 1,2,3-triazolium nitrate, 1-amino-1,2,3-triazolium nitrate, 3,4,5-triamino-triazolium nitrate, and 3,4,5-triamino-triazolium dinitramide.

Chowdhury and Thynell [61] determined the decomposition products of 4ATN subjected to 340 °C, a temperature similar to that applied to 4ATCl, to be N2O, H2O, HNO3, 1TA, and N2. Very low quantities of HCN were detected in the gas phase compared to 4ATCl. The early evolution of HNO3 and 1TA, and the lack of HCN among the gas phase species, which is a by-product of de-composition of 4AT, led to the reversal of the initiation pathways compared to the halide salt. In case of 4ATN, the primary pathway was proposed to be the scission of the amino group from the tria-zolium ring to form HNO3 and an imino radical. The acid oxidises species formed by the imino radical through the ionic route, shown in Fig. 33, or the radical driven route, shown in Fig. 34. The forma-tion of the reactive species NH2NO3, which promptly decomposed to form HNO and HNO2, could not be ruled out, and is shown in Fig. 35. The variation of the decomposition pathways between the halide salt and the nitrate salt was expected to be due to the differ-ence between the basicity of the nitrate anion over the chloride anion, as well as the physical proximity of the chloride anion and the ring N1 hydrogen, as opposed to the proximity of the nitrate anion to the amino group. A secondary pathway involving the de-protonation of the cation to form 4AT and HNO3 was also sug-gested in Fig. 36.

Thermolysis of Me4ATN and Me1ATN was analysed by Chowdhury and Thynell [62] to determine the effect of alkylation of the ring N1 position. The products determined during the decom-position of Me4ATN at 320 °C were primarily 1MeTA, N2O, H2O,

NH2I +

N

NN

H

H

N

NN

H2N

H

H

N

NN

H

HH2N

CH3I +

I-

NH2I + NH3 + Or Or

N

NN

H

HN

NN

I

HN

NN

H

I N

NN

H

H

I

6 NH2I 4 NH3 + 3 I2N2 +

Fig. (31). Proposed reaction pathway for Me4ATI (Chowdhury and Thynell [62]).

NH2I +

N

NN

H2N

H

H

CH3I +

I-

NH2I + NH3 + Or Or

N

NN

H

H

N

NN

H

H

H2N

(Major pathway)

(Minor pathway)

N

NN

H

HN

NN

H

I

N

NN

I

H

N

NN

H

H

I

6 NH2I 4 NH3 + 3 I2N2 +

Fig. (32). Proposed reaction pathway for Me1ATI (Chowdhury and Thynell [62]).


HNO3, N2, and smaller quantities of CO2, HCN, and HNCO. Methyl nitrate was not detected by both of the diagnostic tools. The decomposition was hypothesised to be initiated in a fashion similar to that of Me4ATI and 4ATN, with the formation of NH2ONO2, as shown in Fig. 37. The path based on imino radical formation with the simultaneous formation of HNO3 was not suggested due to the slower evolution of HNO3 compared to 4ATN. NH2ONO2 was expected to form N2O, H2O, and N2 through reactions with

Me4ATN. The compound methyltriazolium nitrate formed in the process was expected to decompose to form HNO3, and 1MeTA. The decomposition of Me1ATN at 340 °C was found to produce a similar species profile, with 3MeTA being detected. While the pathway leading to the demethylation at the ring N3 position was active for the corresponding halide salt, the lack of methyl nitrate led to the proposition of similar reaction pathways as Me4ATN, as seen in Fig. 38. Thus the methylation of the ring nitrogen atom was not found to influence the attack of the nitrate anion on the amino group.

The thermolysis of 4ATN in the condensed phase was studied by Li and Litzinger [63] at high heating rates. The decomposition products were 1TA, 4AT, NO2, N2O, H2O, HCNNH, HNNH, N2,HCN, and NH3. The quantification of various species demonstrated the formation of large quantities of 4AT during the initial period, followed by its decomposition to form 1TA, which led to the de-termination of deprotonation at the ring N1 atom to be the primary reaction pathway, shown in Fig. 39. Deprotonation, as opposed to dealkylation, was also found to be the primary pathway for di-substituted imidazolium nitrates at high heating rates. Nitric acid

+NO3 NH + HNO3

N

NNH

H

HN

NHN

H

H2N

H

H

NH + HNNHNH

NH + NNH + NH2HNNH

NH + N2 + NH2NNH

NH2 + NH2 NH + NH3

NH2 N2 NH3NNH ++

2 HNO3 H2O + NO2+ + NO3

NO2+

+ NH3 N2O + H2O + H+

H+ + NO3 HNO3

Initiation

Propagation

Termination

Fig. (33). Proposed primary reaction pathway for 4ATN by ionic route. (Chowdhury and Thynell [61]).

+NO3 NH + HNO3

N

NNH

H

HN

NNH

H2N

H

H

NH + HNNHNH

NH + NNH + NH2HNNH

NH + N2 + NH2NNH

NH2 + NH2 NH + NH3

NH2 N2 NH3NNH ++

HONO2 OH + NO2

OH + NH3 NH2 + H2O

NH2 + NO2 NH2NO2

NH2NO2 H2O + N2O

Initiation

Propagation

Termination

Fig. (34). Proposed primary reaction pathway for 4ATN by radical-driven route (Chowdhury and Thynell [61]).

+NO3 NH2NO3

N

NNH

H

HN

NHNH

H2N

H

H

NH2NO3 HNO + HONO

2 HNO H2O + N2O

2 HNO2 H2O + NO + NO2

Fig. (35). Proposed alternate reaction pathway for 4ATN (Chowdhury and Thynell [61]).


produced in the initiation reaction was found to partially evaporate and partially react in the condensed phase to form H2O and N2O, in a pathway analogous to the low temperature decomposition of am-monium nitrate as shown in Fig. 40. The formation of 1TA and ammonia was explained by the displacement of the amino group on 4AT by a proton, and the formation of a NH2 radical, which reacts with another 4AT molecule to form ammonia, shown in Fig. 41.

Ab initio quantum chemistry calculations at the MP2 level of theory using the 6-31++G(d,p) basis set of a gas-phase anion-cation pair to predict deprotonation in energetic ionic liquids formed by the 1,2,4-triazolium cation family was carried out by Schmidt et al.[64]. Deprotonation studies with the parent cation and 4AT by ge-ometry optimization indicate that deprotonation from the ring N1

position is a more energy-intensive step during thermal decomposi-

+NO3 HNO3

N

NN

H

HH2N

N

NNH

H2N

H

H

HNO3 OH-+ NO2+

NO2+ +

N

NN

H

HH2N

N

N

N

H

H

N+N

O

O

H

HN

N

N

H

H

NN

O

O

H

+ H+

+ H+OH- H2O

N

N

N

H

H

NN

O

O

H

N

N

N

H

H

NN

O

OHN

NN

H

H

N2O + +OH

N

NN

H

H

+N

NN

H

HH2N

HN

NN

H

H

+N

NN

H

HHN

N

NN

H

H

+OH

N

NN

H

HHO

Fig. (36). Proposed secondary reaction pathway for 4ATN (Chowdhury and Thynell [61]).

+

NO3

ONO2H2N

N

NN

H

H

N

NN

H2N

H

H

N

NN

H

HH2N

+CH3ONO2

+ONO2H2N NO3

N

NN

H

H

H

+ +HNO

+HNO NO3

N

NN

H2N

H

H

NO3

N

NN

H

H

H

+

N2O + H2O

N2 + H2O

NO3

N

NN

H

H

H

HNO3 +

N

NN

H

H

NO3

N

NN

H2N

H

H

Fig. (37). Proposed reaction pathway for Me4ATN (Chowdhury and Thynell [62]).


tion, with an energy requirement of 927 kJ/mol, while the require-ment is 901 kJ/mol for deprotonation from the N4 position for the triazolium cation. The value for deprotonation at the N1 position for the amino-triazolium cation was found to be 941 kJ/mol, although the energy required to abstract the NH2 group from the cation was not reported. The factors responsible for deprotonation were small energy barriers and spontaneous proton-transfer from ionic dimers to form neutral pairs.

The thermal decomposition of several compounds such as as 1,2,3-triazolium nitrate, 1-amino-1,2,3-triazolium nitrate, 3,4,5-triamino-triazolium nitrate, and 3,4,5-triamino-triazolium dinitra-mide was studied using TGA data at different heating rates and the Kissinger method of model-free non-isothermal kinetics [65-68]. As seen in Table 4, the activation energies and the pre-exponential

factors determined for the compounds were found to be reasonably similar for the nitrate salts. However, the activation energy and the pre-exponential factor of decomposition of the dinitramide salt was found to be quite high.

9. DECOMPOSITION OF TETRAZOLIUM-BASED IONIC COMPOUNDS WITH HALIDE ANIONS

Owing to their high energy content and possibility of formation of cooler flames, aided by the profusion of molecular nitrogen, among other products, compared to imidazolium and triazolium-based compounds, tetrazolium-based compounds have been widely tested as burn-rate enhancers as well as monopropellants and explo-sives by the propellant community. Studies on halide salts of tetra-

+

NO3

N

NN

H2N

H

H

+CH3ONO2

N

NN

H

H

N

NN

H2N

H

H

ONO2H2N

+ONO2H2N + +HNO

+HNO +

N2O + H2O

N2 + H2O

HNO3 +

NO3

N

NN

H2N

H

H

NO3

N

NN

H

H

H

NO3

N

NN

H2N

H

H

NO3

N

NN

H

H

H

NO3

N

NN

H

H

H

N

NN

H

H

Fig. (38). Proposed reaction pathway for Me1ATN (Chowdhury and Thynell [62]).

N+

N

NN

H

H

H

H

H

NO3~ N

N

NN

H

H

H

HHNO3

Fig. (39). Possible initiation mechanism: deprotonation at N1 and proton transfer forming 4AT and HNO3 (Li and Litzinger [63]).

N+

N

NN

H

H

H

H

HNO3~ N

N

NN

H

H

H

HHNO3+

+ -+

+ ++

N

N

NN

H

H

H

H+

++ other products

2HNO3 H2NO3 NO3

H2NO3 H2O NO2

NO2 N2O

Fig. (40). Formation of oxygen-containing species from radical-driven chain reactions (Li and Litzinger [63]).


zoles, although few, reveal interesting aspects of their thermal de-composition under both low and high heating rates. Despite their non-energetic nature, halide salts of 5-amino-1H-tetrazole were found to possess higher burning rates compared to the parent mole-cule, and the improvement of burning rates was a function of the anion. The reaction mechanisms of thermolysis of the following halide salts were identified in the literature - 5-amino-1H-tetrazolium X (where X = Cl, Br, and I) (5ATZX); 2-amino-4,5-dimethyl-tetrazolium iodide (2AdMTZI) and 1-amino-4,5-dimethyl-tetrazolium iodide (1AdMTZI).

Brill and Ramanathan studied the thermal decomposition of 5ATZX salts using T-jump/FTIR spectroscopy by subjecting the sample to heating rates of 2000 K/s, and identifying the products from 350 to 450 °C [69]. It was interesting to note that thermolysis of the neutral molecule 1-amino-1H-tetrazole (1ATZ) above 200 °C proceeded with the expulsion of HCN and N2 from the ring leaving N2H2 to produce NH3 and N2. But the shift of the amino group to the ring carbon to form 5ATZ prompted the scission of two weak ring N-N bonds to form HN3 and NH2CN as the principal products. As the temperatures were increased, NH2CN trimerized to form the cyclic azine, melamine. This is interesting to note since the com-pounds discussed henceforth might produce either of these two compounds as a by-product.

The products formed during the decomposition of 5ATZCl were HCl, NH4Cl, HN3, HCN, and dicyandiamide. Although N2

could not be detected, it was expected to be present. The concentra-tion of HCN was found to decrease with increasing temperatures, and that of HN3 increased, leading to the postulation of two differ-ent reaction pathways, as shown in Fig. 42. The first was the disso-ciation of the cation to form molecular nitrogen and a reactive spe-cies, which decomposed spontaneously to form HCN, NH2CN, N2,and the corresponding protonated anion, HCl. The second pathway involved deprotonation of the ring hydrogen and subsequent disso-ciation of 5-amino-tetrazole (5ATZ) into HN3 and NH2CN, which dimerized to dicyandiamide or trimerized to melamine. The first channel of reaction is favoured over the second in the order of I� >Br� >Cl�, which is the same order as the basicities of these ani-

ons. The burning rates of the salts were found to be dependent on the extent to which the first channel, which was hypothesized to be more exothermic than the second channel, dominates over the sec-ond channel. 5ATZI was found to possess the fastest burning rates among the compounds, owing to the prevalence of the first channel.

HN

N

NH+

NNH2

-

+ NH2

HN

ClHN

NH2

HN

ClHN

2

HN

N

NH+

NNH2

-

+

HN

N

NN

NH2

Cl

N2

NH4Cl + HCl + HCN + NH2CN + N2

Cl

HCl

Fig. (42). Thermal decomposition pathways of 5ATZCl (Brill and Ramana-than [69]).

The confined rapid thermolysis of 2AdMTZI and 1AdMTZI at heating rates of approximately 2000 K/s was studied by Chowdhury and Thynell [70] to provide insights into the decomposition of their energetic counterparts, and to understand the effect of the position of the amino group on the ring. The free energies of the reactions were also calculated using DFT theory using the B3LYP functional and the 6-31++G** basis set, except for iodine, for which the 6-311G* basis set was used. The gaseous products identified during the decomposition of 2AdMTZI conducted at 320 °C were CH3I, 5-methyl-tetrazol-2-amine (2A5MeTZ), N2, CH3NC, and methyl-methanimine. As shown in Table 5, although the abstraction of a ring methyl group and a ring amino group are both energetically favourable, the second pathway was relegated to a minor one due to absence of NH2I or NH3 among the products. 2A5MeTZ, formed as

N

N

NN

H

H

H

H

HN

N

N

H

H

H

N

H

H

N

H

H

N

N

NN

H

H

H

H

N

N

NN

H

H

H

NH3

Fig. (41). Formation of triazole and ammonia from neutral amino-triazole (Li and Litzinger [63]).

Table 4. Decomposition of triazolium based compounds.

Compound Activation energy (kJmol-1) Pre-exponential factor (s-1) Reference

1,2,3-triazolium nitrate 133.8 3.80�1014 [65]

1-amino-1,2,3-triazolium nitrate 117.0 ± 7.0 2.51�1012 [68]

3,4,5-triamino-triazolium nitrate 137.2 1.07�1010 [67]

3,4,5-triamino-triazolium dinitramide 165.6 1.09�1018 [66]


a product with CH3I, decomposed through a ring nitrogen expulsion to form the other detected species, as shown in Fig. 43.

The decomposition of 1AdMTZI was found to proceed at lower temperatures, and the species detected at 250 °C were NH3, CH3N3,CH3I, 1A5MeTZ, N2. As shown in Table 6, the decomposition of 1AdMTZI, as opposed to 2AdMTZI, was found to proceed through multiple reaction channels and was not dominated solely by the methyl group transfer involving the cleavage of a ring-exo N-C bond, forming CH3I and 5-methyl-tetrazol-1-amine (1A5MeTZ). The other two important pathways were initiated by the extraction of a hydrogen atom by the amino group from the neighbouring

methyl group to form NH3 and 5-iodomethyl-1-methyl-1H-tetrazole (5IdMeTZ), and by the removal of N2 from the ring, with the re-maining unstable species decomposes rapidly in the condensed phase to produce smaller products. The decomposition of 1A5MeTZ through the N2-expulsion channel was confirmed based on the products observed and shown in Fig. 44.

10. DECOMPOSITION OF TETRAZOLIUM-BASED IONIC COMPOUNDS WITH ENERGETIC ANIONS

Thermal decomposition studies on energetic tetrazolium-based salts were based on various substituted tetrazolium cations, with the

Table 5. Theoretical reaction enthalpies and free energies for 2AdMTZI at B3LYP/6-31++G** level (Chowdhury and Thynell [70]).

No. Reactions �HR

(kJ/mol) �GR (kJ/mol)

1.

N N

N N+

H2N

I- +CH3IN N

NN

NH2

5-methyl-tetrazol-2-amine(2A5MeTZ)

�41.6 �94.9

2.

N N

HN N+

H2N

I-

+

N N

NNNH2I

1,5-dimethyl- tetrazole(dMeTZ)

�53.9 �105.0

3.

N N

N N+

H2N

I-

N N

NN

NH2

+CH3I


151.6 105.3

+N2

CH3NC

N N

NN

NH2

N

N-

C+

H3C

NH2

N

N-

C+

H3C

NH2

+

H3C NH

H3CN

CH2

NH

[Low Temperature]

[High Temperature]

+N2

+N2 HH

CH3CN + H2NN

NN

+N2 NH3 [Ammonia not observed]

[m/z = 43]

2A5MeTZ

[Nitrilimine]

5 2H2NN

NN

3

Or

Or

Fig. (43). Considered secondary reaction pathways for 2AdMTZI (Chowdhury and Thynell [70]).


Table 6. Theoretical reaction enthalpies and free energies for 1AdMTZI at B3LYP/6-31++G** level (Chowdhury and Thynell [70]).

No. Reactions �HR


1. +CH3I

N N

N N+H2N

I- N N

NN

H2N


�71.9 �65.6

2.

N N

N N+H2N

I-

+NH3

N N

NN

I

5-(iodomethyl)-1-methyl-1 -tetrazole(5IdMeTZ)

H

�74.2 �118.7

3.

N N

N N+H2N

I- H+

N NHNN2 +

I-

�123.6 �173.9

4.

N N

N N+H2N

I-

1,5-dimethyl-tetrazole(dMeTZ)

N N

NN+NH2I

�33.2 �84.7

5.

N N

N N+H2N

I-

+NH3

N N

NN

I

95.9 51.2

6.

N N

N N+H2N

I- N N

C NN

H2N

+CH3I

1-amino-4-methyl-tetrazol-5-ylidene(1A4MeTZ)

109.1 60.2

+ N2N N

NN

H2N

N

N+

N

H2N

N-

N

N

H2N

NH3

H2C

N

N

+ H2NN

NN

[Not Observed]

CH3CN

H2C

N

N

N

N

CH2

(1A5MeTZ)

N N

NN

I

N2 +

N

NI

CH3I

[Condensed Phase]

[Dimerization,Condensed Phase]

H3CN

NN

+ C NH2C

I

[Condensed Phase]

[Nitrene]

H2CN

N

Fig. (44). Considered secondary reaction pathways for 1AdMTZI (Chowdhury and Thynell [70]).


NN

N+NNH2

NH2

Me -

+

NN

NN

NH

NH2

Me

160 - 185 o

105 - 160°C

H2N NH

CN

+[H2N=N]+

HN

N N

N

N

N

NHNH

HN

NH2

H2N

NH2

crosslinkedproducts

2d2

34

87

6

5

+

9

N3

HN3

C

HCN

-N2

-NH3

-N2

-NH3

I

HN3(g)NH3(g)NH4N3(solid)

II

MeN3

Fig. (45). Proposed decomposition pathway of MeDATZA (Fischer et al. [72]).

substituent groups being amino and methyl, paired with anions such as nitrate (N), dinitramide (DN), and azide (A). The compounds that were identified were 5ATZN, 5ATZDN, 1,5-diamino-tetra-zolium nitrate (DATZN), 5-amino-1-methyl-tetrazolium dinitra-mide (MeATZDN), 2AdMTZN, 1AdMTZN, 1,5-diamino-4-methyl-tetrazolium nitrate (MeDATZN), 1,5-diamino-4-methyl-tetrazolium azide (MeDATZA), 1,5-diamino-4-methyltetrazolium dinitramide (MeDATZDN). One study was focussed on the decom-position of an oxygen-rich energetic coordination compound based on cadmium, [Cd(DAT)6](ClO4)2.

In one of the earliest studies, Ma et al. [71] analysed the de-composition behaviour of 5ATZN by TGA-DSC techniques and by detecting the species in the residue post decomposition. The de-composition was found to occur in several stages, with the first exothermic step postulated as a deprotonation reaction with HNO3

and 5ATZ being the initial products. HNO3 was expected to de-compose to form H2O and NO2. However, it was odd to observe that the authors did not propose nitrogen abstraction from the ring as a decomposition pathway, as that would have explained the ob-served 18% mass loss. The second exothermic decomposition step was expected to be the decomposition of 5ATZ to form HN3 and NH2CN, which trimerizes to form melamine in the condensed phase. Further mass loss was explained by the formation of melem from melamine and melon from melem by abstraction of ammonia. The activation energy and pre-exponential factor associated with the first step of decomposition was calculated as 311 kJ/mol and 8.3 � 1032 s-1 using Kissinger’s method, which were unrealistic for a tetrazole-based compound, and are expected to be the artefacts of a kinetic compensation effect.

The slow thermal decomposition characteristics of derivatives of 1,5-diaminotetrazole, MeDATZN, MeDATZDN, and

MeDATZA were examined using TGA and DSC, combined with the model-free approach of estimation of activation energies by Fischer et al. [72]. The gaseous products were identified by IR spectroscopy and MS, while the residues were analysed by MS and NMR techniques. The activation energies of MeDATZN, MeDATZDN, and MeDATZA, were found to be 130.1, 137.7, and 107.6 kJ/mol respectively, identifying the azide salt as thermally more labile than the other two.

The decomposition products of MeDATZA were identified as HN3, NH4N3, HCN, NH3, MeN3, N2, traces of 1,2,4-triazole (TA), and other cross-linked products. MeDATZA decomposed through deprotonation to form 1-amino-4-methyl-5-imino-tetrazole and the corresponding acid HN3. The decomposition temperatures were high enough to cause ring fracture of the tetrazole formed during the initiation step and led to the formation of smaller fragments, including MeN3 and aminocyanamide, which decomposed further to form HCN, as well as trimerized to form 2,4,6-trihydrazino-1,3,5-triazine, giving rise to the cross-linked products and ammonia subsequently. The reaction scheme is shown in Fig. 45.

The thermolysis of MeDATZN led to the identification of Me-ONO2, HNO3, NH3, HCN, N2, CO, CO2, N2O, H2O, CH2O, CH3OH, MeN3, TA, melamine, 1,3,5-triazine, and cross-linked products. Decomposition involved primarily methyl group transfer to form MeONO2, along with a secondary pathway of proton trans-fer to form HNO3. MeONO2 was expected to decompose under the temperatures involved to form CO, CO2, NO, NO2, H2O, CH3OH, and H2CO, among other products. The other by-product, 1,5-diaminotetrazole (DAT), readily produced nitrogen and an unstable nitrene, which subsequently produced NH3, N2, and HCN. The nitrene was also expected to form melamine through an intermedi-ate. The minor quantities of 1-amino-4-methyl-5-imino-tetrazole


formed decomposed in a fashion similar to the one detailed for MeDATZA. The triazine was formed by in-situ polymerization of HCN, as shown in Fig. 46.

The typical products formed during the decomposition of MeDATZDN were N2O, MeN3, MeONO2, HCN, NH3, H2O, and 1,3,5-triazine. As seen in Fig. 47, the initiation reaction was identi-fied to be a deprotonation to form dinitraminic acid, HN(NO2)2, and

1-amino-4-methyl-5-imino-tetrazole and. HN(NO2)2 decomposes to form N2O and HNO3, and HNO3 promptly reacts with 1-amino-4-methyl-5-imino-tetrazole producing its nitrate salt, which decom-poses in a manner discussed earlier. In case of these salts, it was interesting to observe that the reaction mechanism did not proceed through the amino group, and the deprotonation pathways were found to be active for MeDATZA and MeDATZDN at low heating rates despite the presence of methyl and amino groups.

O

NO2

Me

NN

NN

NH2

NH2

+185 - 250°C

NN

N+NNH2

NH2

Me -

NN

NN

NH

NH2

Me

+

HN

N N

+ + crosslinkedproducts

H2N

N

N NH2N

N

N

NH2H2N

NH2

+ NH3 + HCNN

N

N

111

2b

210

12 136

3

14

3(g) + HNO

3(g)

> 200 o

+ H2O (g)

exp.

15

11

15 16

I

NO3

HNO3

-N2

N2

NH NH4NO3 (solid)

MeONO2 H2CO, CH3OH, H2O, CO, NO + NO2

C

NH4NO3 N2O(g)

II

III

IV

V

MeN3

Fig. (46). Proposed decomposition pathway of MeDATZN (Fischer et al. [72]).

NN

NN

NH

NH2

Me

+150 - 250°C

NN

N+NNH2

NH2

Me

NN

NN

NH2

NH2

+ MeN(NO2)2

+ NN

N+NNH2

NH2

Me

decomposition

+ HNNO2 (g)

17

2

2c

118

10

2b

17 19 20

16

I

N2

O

HNO3

NO3-

NO2(g)

II

III

HN(NO2)2

N(NO2)2 -

HN(NO2)2

Fig. (47). Proposed decomposition pathway of MeDATZDN (Fischer et al. [72]).


The nitrate salts of 2AdMTZ and 1AdMTZ were subjected to confined rapid thermolysis/FTIR spectroscopy/TOFMS by Chowd-hury and Thynell [70]. The products detected during the decompo-sition of 2AdMTZN at 300 °C were CH3ONO2, 2A5MeTZ, CH3NC, H2O, CO2, HCN, CH3CHO, and a mixture of CH3CNO and CH3NCO. The primary reaction pathway was determined to be one similar to that of 2AdMTZI, with the methyl group forming CH3ONO2 and 2A5MeTZ, as shown in Table 7. Subsequent reac-tions between CH3ONO2 and 2A5MeTZ replace the methyl group on 2A5MeTZ to form 2-amino-5-methoxy-2H-tetrazole, as ob-served during the thermolysis of di-alkyl-substituted imidazolium nitrates. The methoxy-tetrazole undergoes a decomposition process

similar to 2A5MeTZ to form the species CH3NCO and CH3CNO through nitrogen elimination. Besides, CH3ONO2 and CH3ONO partially decompose to form smaller products. The reactions are detailed in Fig. 48.

1AdMTZN decomposed at 250 °C, which was 50 °C lower than the decomposition temperature of 2AdMTZN, forming a variety of products, such as CH3ONO2, H2O, NO, NO2, N2O, N2, 1A5MeTZ, CH3CNO, CH3NCO, CH3NC, and smaller quantities of CO, CO2,CH4, and CH3OH. As observed from the decomposition pathways of the iodide salt, three major pathways—a nucleophilic transfer involving the methyl group attached to the ring nitrogen to form 1A5MeTZ and CH3ONO2 (reaction 1), ammonia formation by the

CH3NC

N N

NN

NH2

N

N-

C+

H3C

NH2

++N2 +N2 H2

CH3CN + H2NN

NN2A5MeTZ

CH3ONO2 + +

+N2 N2 HH+

H3CN

CO

H3C NO

+

CH3ONON N

NN

NH2N N

NN

O

NH2

H3C

N N

NN

O

NH2N N-

C+O

NH2

H3COr

+N2 NH3

[Ammonia not observed]

5 2H2NN

NN

3

Fig. (48). Considered secondary reaction pathways for 2AdMTZN (Chowdhury and Thynell [70]).

Table 7. Theoretical reaction enthalpies and free energies for 2AdMTZN at B3LYP/6-31++G** level (Chowdhury and Thynell [70]).

No. Reactions �HR (kJ/mol) �GR (kJ/mol)

1.

N N

N N+

H2N

+CH3ONO2

N N

NN

NH2


NO3

�14.3 �67.9

2. +

N N

NN

1,5-dimethyl- tetrazole(dMeTZ)

N N

N N+

H2N

NH2ONO2NO3

21.2 �27.9

3.

N N

NN

NH2

+


N N

N N+

H2N

CH3ONO2

NO3

179.0 132.4


amino group through a proton abstraction from the neighbouring methyl group (reaction 2), as well as ring nitrogen elimination (re-action 3), were all found to be thermodynamically feasible proc-esses, and are shown in Table 8. The subsequent reactions involv-ing 1A5MeTZ and CH3ONO2 are shown in Fig. 49. The smaller molecular weight species, namely N2O, H2O, CO, NO, CO2, and NO2 were formed by the oxidation of NH3 by methyl nitrate. A portion of CH3ONO2 reacts through the formation of 1-amino-5-methoxy-1H-tetrazole to form CH3NCO and CH3CNO, forming CH3ONO in the process. Besides these primary reactions, numerous other secondary reactions involving the intermediate nitrenes, CH3ONO2, and CH3ONO also take place to form the simpler prod-ucts.

The decomposition of the energetic coordination compound, Cd(DAT)6](ClO4)2, was studied by Cui et al. [73] using TGA-DSC techniques. The residues of decomposition were analysed further by FTIR spectroscopy. The process was found to occur in three distinct steps, with the detection of Cd(ClO4)2, Cd(CO3)2, and a [�CO�NH�] polymer in the first stage, Cd(NCO)2 in the second stage, and CdCl2 in the third stage. The possible reaction mecha-nism is shown in Fig. 50. The activation energy and pre-exponential factor associated with the first step of decomposition was calculated

as 199.7 kJ/mol and 7.4 � 107 s-1 using Kissinger’s method, which was high for a tetrazole-based compound.

The thermal decomposition of several amino-methyltetrazolium salts, DATZN, 5ATZDN, MeATZDN, MeDATZDN, and MeDATZA was studied by Piekiel and Zachariah [74]. The heating rates applied were approximated to be 326,000 K/s and the sample temperatures achieved were 1270 K, which were both higher than any of the previous studies on tetrazolium-based salts. 5ATZDN was found to decompose through a scission of the tetrazolium ring and its subsequent protonation. The tetrazole ring was postulated to decompose through both the possible mechanisms noted earlier, through a nitrogen abstraction to form NH2NCNH and through HN3

elimination to form NH2CN. Conclusive evidence could not be found in favour of the dominance of either pathway. The dinitra-mide ion was expected to decompose through two pathways - the formation of NO2 at lower temperatures and formation of NO and N2O at higher temperatures. The second compound, MeDATZDN, was found to decompose through the expulsion of the methyl and the amino group from the tetrazolium cation, forming aminotetra-zole, which decomposed through a ring nitrogen expulsion in a fashion similar to 5ATZDN. The dinitramide anion was found to decompose through similar pathways as well, at a faster rate com-

Table 8. Theoretical reaction enthalpies and free energies for 1AdMTZN at B3LYP/6-31++G** level (Chowdhury and Thynell [70]).

No. Reactions �HR


1. +

N N

N N+H2N

N N

NN

H2N


NO3 CH3ONO2

4.1 �44.6

2. +NH3

N N

NN

O2NON N

N N+H2N

NO3

�66.3 �108.3

3.

H+

N NHNN2 +

N N

N N+H2N

NO3

NO3

�67.9 �117.7

4.

1,5-dimethyl-tetrazole(dMeTZ)

N N

NN+NH2ONO2

N N

N N+H2N

NO3

36.1 �13.85

5. +NH3

N N

NN

O2NO

N N

N N+H2N

NO3134.7 90.8

6.

N N

N N+H2N

N N

NN

H2N

+CH3ONO2

1-amino-4-methyl-tetrazol-5-ylidene(1A4MeTZ)

NO3

130.4 81.1


pared to 5ATZDN. This pathway is radically different than the one predicted earlier at lower heating rates, involving deprotonation of the cation. The decomposition pathways of the azide compound, MeDATZA, was found to be similar to that of MeDATZDN.

MeATZDN was found to decompose through elimination of HN3, instead of the ring N2-elimination route, which was attributed to the presence of the methyl group on the ring, and the associated asymmetric nature of the cation. Additionally, only the low tem-perature decomposition pathway of the dinitramide anion was found to be active in this case, as the decomposition was completed at low temperatures. DATZN, being similar in nature to MeATZDN in terms of the asymmetry of the cation, was found to decompose in a similar manner.

Based on the concluded discussion on the energetic and non-energetic ionic salts with imidazolium, triazolium, and tetrazolium-based cations, it is evident that the initiation reaction is a strong function of the applied temperature, heating rates, and the substitu-ent groups on the cations. The possible pathways involve deproto-nation, dealkylation, deamination, as well as direct ring scission of

the cation, and no clear trend could be observed regarding the choice of the initiation pathway.

11. CONCLUDING REMARKS

Although ionic compounds have not yet been adopted into the mainstream as far as applications in the field of high energy materi-als are concerned, as evinced by the huge number of synthesized energetic ionic salts in the recent past, there is a tremendous scope for further exploration and analysis of these compounds to extract information about their energetic characteristics. Previous research on synthesis and determination of thermo-physical as well as ener-getic properties of energetic ionic compounds has been extensively reviewed by several researchers. The current review is expected to complement the previous work by including proposed thermal de-composition pathways of some of the promising compounds, thus providing the groundwork for better understanding of their capabili-ties during combustion. The review also is expected to elaborate the current status of research on pyrolysis of energetic ionic salts - both through traditional as well as recent experimental techniques, such

CH3ONO2 + +N N

NN

O

H2N

+N2 N2 HH+

H3CN

CO

H3C NO

N N

NN

H2N

N N

NN

O

H2N

N

NO

H2N

+ N2N N

NN

H2N

N

N+

N

H2N

N-

N

N

H2N

NH3

H2C

N

N

+ H2NN

NN

[Not Observed]

CH3CN

(1AMeTZ)

N N

NN

O2NO

N2 +

N

NO2NO

CH3ONO2

NN

N+

C N

O2NO

[Condensed Phase]

+

CH3ONO

2HCN

N

N

N

N

[Condensed Phase]

[Dimerization,

Condensed Phase]

H2CN

N

Or

Fig. (49). Considered secondary reaction pathways for 1AdMTZN (Chowdhury and Thynell [70]).

[Cd(DAT)6](ClO4)2 + Cd(CO3)2 + [-CO-NH-] polymer + gas products

+ gas products + gas productsCd(NCO)2 CdCl2

Cd(ClO4)2

Fig. (50). Considered reaction pathways for Cd(DAT)6](ClO4)2 (Cui et al. [73])


as TGA-DTA, DSC, flash pyrolysis, confined rapid thermolysis, laser ablation, coupled with various diagnostic techniques as FTIR spectroscopy, TOFMS, GC, VUV-PI-TOFMS; and computational techniques such as ab initio methods, molecular dynamic simula-tions, reactive force field methods, etc.

From the information compiled in this review it is clear that while experimental work on the decomposition of energetic ionic liquids provides the product composition, computational tools are absolutely necessary to elaborate the detailed reaction mechanism along with the respective rate parameters. Hence, experimental techniques should be combined with chemical kinetic estimations to provide at least global reaction mechanisms, or at best semi-global reaction mechanisms with the associated kinetic parameters. As a future course of action, researchers need to combine experimental and computational approaches such that they complement each other to elucidate the decomposition pathways. It is also evident that there is a necessity to continue research on the decomposition analysis of energetic ionic salts, since the reaction schemes which have been reported so far for most energetic ionic compounds are only estimates of how the parent molecules lead to the observed products. If the predictive capability of various combustion simula-tion models is to be improved, we need to have at least a global reaction mechanism along with the corresponding rate parameters if not a semi-global one for each compound. Detailed or reduced chemical kinetics mechanisms are currently available for very few compounds, primarily due to constraints posed by computational resources, but may become the prevalent norm in the future.

CONFLICT OF INTEREST

The author(s) confirm that this article content has no conflict of interest.

ACKNOWLEDGEMENTS

Declared none.

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