Products of Thermal Decomposition of Guanidinium Perchlorate

R. N. Isaev, Yu. A. Zakharov, N. I. Odin

(Presented at the scientific seminar of the Department of Radiation Chemistry)

The thermal decomposition of guanidinium perchlorate (GP) has been studied insufficiently; publications on this question are limited to works [1–7]. The authors of [4] studied some physicochemical properties of GP; in [1, 6] the composition of the products of thermal decomposition is given; in [1, 5, 7] variants of the mechanism of thermal decomposition are proposed; and in [3] the effect of additives on the ignition delay time of GP was studied.

The information presented in [1] and [6] on the composition of the products of thermal decomposition of GP differs somewhat from one another with respect to the nitrogen- and chlorine-containing products and therefore requires clarification. The views of the authors of [1] and [5] on the nature of the initial stage of GP decomposition also do not coincide.

The aim of the present work is to clarify the composition of the final products (mainly gaseous ones) and to identify the intermediate products of the thermal decomposition of GP.

Experimental Procedure

The products of thermal decomposition were analyzed by mass spectrometry using MX-1302 and MI-1305 mass spectrometers according to two different procedures.

In the first case, GP decomposed in a specially designed reaction vessel that was part of the inlet system of the MX-1302 mass spectrometer [8]. The gaseous decomposition products entered the analyzer directly through a special diaphragm providing molecular flow; simultaneously with the analysis, manometric (using ...

Bourdon gauge) the kinetics of decomposition of GP were observed. In this case the final decomposition products are recorded, including those that are the result of subsequent secondary interactions between the evolved gases, gaseous and solid products of thermal decomposition. Analysis of the products of thermal decomposition was carried out both during the course of decomposition at one fixed temperature and as a function of temperature at one degree of conversion.

In the second case, GP was decomposed directly in the “furnace” of the ion source of an MI-1305 mass spectrometer [9]. The gaseous decomposition products, both final and intermediate, were recorded by the instrument.

The conditions of thermal decomposition in this case were such (low pressure \(2 \cdot 10^{-6}\) torr, GP sample less than \(10^{-3}\) g, continuous evacuation of the source) that the probability of secondary reactions of the products with one another, as well as reactions of the decomposition products of GP with GP, became insignificant.

The operating conditions were as follows: on the MX-1302, accelerating voltage \(U_y = 1.5\) kV, ionizing voltage \(= 70\) V, emission current \(= 2\) mA, sample of substance \(\sim 200\) mg; on the MI-1305, \(U_y = 4\) kV, \(U_i = 50\) V, \(I = 1.5\) mA.

RESULTS OF THE EXPERIMENTS

Analysis on the MX-1302 showed that the main gaseous products of the thermal decomposition of GP in the temperature range \(280\text{–}480^\circ\mathrm{C}\) are (Table I): \(H_2O\), \(N_2\), \(CO\), \(N_2O\), \(CO_2\), \(O_2\), \(HCl\), \(CN_2H_2\), \(CN_3H_3\); along with these, \(NO_2\) and \(NH_3\) are present in small amounts. The qualitative composition of the products established by us is identical for most components to the compositions of the products reported in works [1,6]. However, in addition to those found in [1], nitrogen oxides, cyanamide, and guanidine were detected by us in the products.

Glaznov and Makovki [1] represent the change in the qualitative composition of the decomposition products of GP with increasing decomposition temperature by the following two equations:

\[ 2CH_5N_3 \cdot HClO_4 \longrightarrow 2HCl + 5H_2O + CO + CO_2 + 3N_2 \tag{1} \]

\[ 2CH_5N_3 \cdot HClO_4 \longrightarrow 2HCl + 2NH_3 + O_2 + 2N_2 + 2CO_2 + 2H_2O \tag{2} \]

Vertical axis: mol. % in decomposition products
Horizontal axis: \(t^\circ C\)

Fig. 1. Dependence of the content of guanidinium perchlorate decomposition products on temperature.

1. \(N_2\) and \(CO\)
2. \(N_2O\) and \(CO_2\)
3. \(CN_2H_2\)
4. \(O_2\)
5. \(HCl\)
6. \(H_2O\)
7. \(CN_3H_5\)
8. \(NH_3\) and \(NO_2\)

The results of the experiments we carried out did not confirm the authors’ opinion [1]: the qualitative composition of the products remains unchanged both with increasing degree of decomposition at one and the same temperature and with increasing temperature at one and the same degree of decomposition \((\sim 15\%;\) Fig. 1).

With an unchanged composition, in the course of thermal decomposition the relative contents of the individual components in fact also do not change, which may indicate the invariability of the mechanism throughout the entire process. The relative content of the components at different decomposition temperatures changes somewhat (Fig. 1), but the nature of these

Table I

Mass spectrum of gaseous products of thermal decomposition of GP at 350°C (analysis on MX-1302)

does not meet that required on the basis of the assumption [1] of parallel occurrence of reactions (1) and (2) and of their relative role at different temperatures.

Analysis of the products from decomposition of GP in the MI-1305 ion source made it possible to detect additional products of the thermal decomposition of GP which are not analyzed on the MI-1302. These include: $\mathrm{Cl_2}$, $\mathrm{NH_4Cl}$, $\mathrm{C_3N_6H_6}$, $\mathrm{HClO_4}$, and chlorine oxides $\mathrm{ClO}$, $\mathrm{ClO_2}$, $\mathrm{ClO_3}$ (Table 2). The presence of melamine, $\mathrm{C_3N_6H_6}$, in the decomposition products makes it possible to assert that, in the process of cyanamide condensation established in [1], the principal product is only melamine, and not a mixture of organic bases—of the melone, melam, ammelin type—as was assumed by the authors of [1].

The absence of chlorine in the decomposition products analyzed on the MI-1302 is apparently due to its participation in the oxidation reactions of ammonia and cyanamides (along with oxygen and chlorine oxides).

DISCUSSION OF RESULTS

As already reported above, the composition of the products of thermal decomposition of GP established by us coincides, for most components, with the data of Glazner and Makovka [1]. The difference consists in the fact that, among the products, we found nitrogen oxides (along with $\mathrm{N_2}$), $\mathrm{NH_4Cl}$, $\mathrm{Cl_2}$, $\mathrm{CN_2H_2}$, and $\mathrm{CN_3H_5}$. The formation of nitrogen oxides during thermal decomposition of GP is to be expected, since the oxidation of ammonia (possibly also $\mathrm{CN_2H_2}$, $\mathrm{CN_3H_5}$, $\mathrm{C_3N_6H_6}$) evidently occurs heterogeneously—in the volume of the liquid phase (GP decomposes in the molten state) or on its surface. In this case the oxidation products of $\mathrm{NH_3}$ are nitrogen and its oxides, and with increasing temperature the amount of the latter increases [10].

The scheme of thermal decomposition of GP proposed in [1] assumes, as the primary stage of the reactions, bond cleavage within the cation:

$$ \mathrm{CN_3H_5 \cdot HClO_4 \longrightarrow CN_2H_2 + NH_4ClO_4,} \tag{3} $$

Further, according to this scheme, the PCA formed thermally decomposes into ammonia and perchloric acid; the latter oxidizes $\mathrm{NH_3}$, cyanamide, and the products of its condensation to the final products:

$$ \mathrm{NH_4ClO_4 \longrightarrow NH_3 + HClO_4} \tag{4} $$

\[ 3CN_2H_2 \longrightarrow C_3N_6H_6 \tag{5} \]

\[ C_3N_6H_6,\ NH_3 + HClO_4 \xrightarrow{O_2} HCl,\ CO_2,\ CO,\ H_2O,\ NH_3,\ N_2 \tag{6} \]

We [7], on the basis of mass-spectrometric data for the products of thermolysis of GP, in contrast to [5], proposed as the primary stage a proton transfer with formation of perchloric acid and guanidine:

\[ CN_3H_6 \cdot HClO_4 \longrightarrow CN_3H_5 + HClO_4 . \tag{7} \]

This process was proposed as the primary one in the thermal decomposition and sublimation of GP in [11]. In the recently published work [5], the authors consider that, according to reaction (7), \(2/3\) of GP decomposes, whereas according to the Glazner and Makovki scheme only \(1/3\) of GP does.

The data of the present work confirm that the decomposition of GP proceeds predominantly according to reaction (7). This is consistent with the detection of guanidine and perchloric acid as intermediate and final products of the thermal decomposition of GP (Tables 1, 2).

Furthermore, investigation of the thermal stability, derivatographic analysis, and mass spectrometry of the sublimation products formed during decomposition of GP showed the identity of the properties of the sublimate and of the initial GP.

On this basis it may be considered that the sublimate consists predominantly (or completely) of GP.

Finally, the similarity, expected from general considerations (and confirmed in [11]), to the thermal decompositions of amines of perchloric acid also permits reaction (7) to be proposed as the principal one, since proton transfer as the first stage of thermal decomposition of PHA has been convincingly demonstrated by various methods in a number of works.

Competing with sublimation is the reaction of thermal decomposition of guanidine:

\[ CN_3H_5 \longrightarrow CN_2H_2 + NH_3 \tag{8} \]

The presence of cyanamide and ammonia in the decomposition products of GP, the low thermal stability of \(CN_3H_5\) (decomposes at \(+250^\circ C\)), and also the ratio established in [11],

\[ \frac{NH_3}{HClO_4} \approx 1 \]

(valid for the initial moments of thermal decomposition), make it possible to consider the occurrence of reaction (8) probable.

Part of the cyanamide condenses according to reaction (6).

A second process, apparently competing with the formation of the GP sublimate, may be the thermal decomposition of perchloric acid. The character and regularities of its course in this case are unknown to us; one may assume (by analogy with the thermal decomposition of PCA [13]) that the decomposition of $HClO_4$ proceeds heterogeneously according to scheme [14], with formation of chlorine oxides as intermediate products, which are also recorded in the mass spectrum of GP (Table 2):

\[ HClO_4 \longrightarrow ClO_3 + OH \tag{9} \]

\[ ClO_3 \longrightarrow ClO_2 + O \tag{10} \]

\[ ClO_2 \longrightarrow ClO + O \tag{11} \]

\[ ClO \longrightarrow Cl^{0} + O \tag{12} \]

\[ 2ClO_3OH + M \longrightarrow M + Cl_2 + H_2O + O_2 \tag{13} \]

The decomposition products of $HClO_4$ evidently also oxidize, heterogeneously, ammonia, guanidine, cyanamide, and the condensation products of the latter (see above).

The considerations presented make it possible to propose, thus, the following scheme as a probable scheme of the thermal decomposition of GP:

\[ CN_3H_5\cdot HClO_4\;(\text{sublimate}) \longrightarrow CN_3H_5\;(\text{gas}) + HClO_4\;(\text{gas}) \]

\[ CN_3H_5\cdot HClO_4 \rightleftarrows CN_3H_5\;(\text{ads}) + HClO_4\;(\text{ads}) \]

\[ CN_3H_5\;(\text{ads}) \longrightarrow CN_2H_2 + NH_3 \]

\[ 3CN_2H_2 \longrightarrow C_3N_6H_6 \]

\[ HClO_4\;(\text{ads}) \longrightarrow \text{decomposition products} \]

\[ CN_3H_5,\; CN_2H_2,\; C_3N_6H_6,\; NH_3 + \text{decomposition products of } HClO_4 \longrightarrow \]

\[ \longrightarrow H_2O,\; N_2,\; N_2O,\; NO_2,\; CO_2,\; CO,\; HCl,\; O_2,\; NH_4Cl \]

A number of questions concerning the proposed scheme (the probability of the occurrence of the parallel reaction (4), the character of the thermal decomposition of $HClO_4$, the possibility of reaction of the products with GP itself, and others) require further experimental verification and refinement.

Table 2

Composition of gaseous products of GP thermolysis
(decomposition in an MI-1305 ion source at 350°C)

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