| Hexogen(RDX) is one of the most important high energetic explosives, the excellent performance makes it widely used in military field, and the research on preparation process of RDX has been a great concern in the field of explosives. Today, the synthetic process of RDX mainly contains nitric acid method and acetic anhydride method. In China, nitric acid method becomes the major production technology. The direct nitration process(hereafter referred to as the direct method) is the synthesis of RDX from urotropine by direct nitration with nitric acid. Based on the current problems to direct nitration of RDX such as low yield, unclear byproducts and reaction mechanism, etc, it is of great significance and necessity to study the possible nitrolysis mechanism during the preparation of RDX by direct method for the high yield and achievement of green process.RDX was synthesized from urotropine by direct nitrolysis with concentrated nitric acid, and then poured into ice-water, RDX was separated out. After RDX was filtered out, waste water was obtained for the following use. Byproducts were enriched from waste water by extraction of ethyl acetate, washed with water to neutral, and concentrated. Silicone column chromatography and thin-layer chromatography(TLC) were applied to separate the stable byproducts. The purity of the byproducts was analyzed by high performance liquid chromatography(HPLC), melting point and TLC. Fourier transform infrared spectroscopy(FTIR),nuclear magnetic resonance(NMR), elemental analysis and mass spectrometry(MS) were used to characterize their structures, which were recognized as 3,5-dinitro-1-oxa-3,5-diazacyclohexane, 1,3,5-trinitro-1,3,5-triaza-cyclohexane(RDX) and 1-(1-nitroxyl-2-hydroxymethyl-2-aza-propyl)-3,5-dinitro-1,3,5-triaza-cyclohexane. The possible nitrolysis mechanism during the preparation of RDX by direct nitration could be analyzed through associating the relationship of urotropine and the byproduct structure with the basic theory of organic reactions.According to the direct method, different factors were investigated to discuss the effects of byproducts. Possible cases of byproducts during synthesis process could be analyzed to provide experimental evidences for the nitrolysis mechanism. The main factors were as follows:(1) material ratios of HA and NA were 1∶11, 1∶8 and 1∶5, the results showed that we obtained four compounds by extraction, compound 2 was 3,5-dinitro-1-oxygen-3,5-diazacyclohexane, compound 3 was RDX and compound 4 was 1-(1-nitroxyl-2-hydroxymethyl-2-aza-propyl)-3,5-dinitro-1,3,5-triaza-cyclohexane. The content of compound 1 was less than that of others at the material ratio was 1∶11 through changing the material ratios. The content of compound 4 was reduced with the decrease of material ratios of HA and NA;(2) mature time were 0 min, 5min, 10 min, 15 min and 20 min, it found that we detected five compounds by TLC at the mature time of 0 min, 5min, 10 min, compound 2 was 3,5-dinitro-1-oxygen-3,5-diazacyclohexane, compound 3 was RDX and compound 5 was 1-(1-nitroxyl-2-hydroxymethyl-2-aza-propyl)-3,5-dinitro-1,3,5-triaza-cyclohexane. When the mature time were 15 min and 20 min, there were just four compounds were detected, because the unstable byproducts may be decomposed with the extend of mature time;(3) extraction agents were ethyl acetate, trichloromethane and diethyl ether, respectively, 3,5-dinitro-1-oxygen-3,5-diazacyclohexane, RDX and 1-(1-nitroxyl-2-hydroxymethyl-2-aza-propyl)-3,5-dinitro-1,3,5-triaza-cyclohexane were detected through the three kinds of extraction agents, but we found the content of RDX was low in diethyl ether because the solubility of RDX was low in it.The structure of 3,5-dinitro-1-oxygen-3,5-diazacyclohexane was determined by an X-ray single crystal diffractometer. The results indicate that the crystal molecular weight is 178.12. It belongs to monoclinic system with space group P121/n1, a = 0.58128(13) nm, b = 1.72389(14) nm, c = 0.71072(6) nm, β = 112.056°, V = 0.66006(16) nm3, Z = 4, DC = 1.792 g·cm–3, μ = 0.17 mm–1, F(000) = 368.0, the final deviation factor R is 0.0397. The DSC-TG technique was used to investigate the thermal behavior of this compound, which showed a pair of sharp peaks at 375.85 K(melting) and 519.05 K(decomposition). In addition, corresponding to the Kissinger method and Flynn-Wall-Ozawa method and the TG data of different heating rates, the kinetics parameter values E and A were obtained. Meanwhile the Coats-Redfern method was applied to study the thermal decomposition mechanism of 3,5-dinitro-1-oxygen-3,5-diazacyclohexane. The results indicate that the compound is a low melting point compound with good stability, its apparent activation and pre-exponential factor calculated by Kissinger equation are 212.32 k J·mol–1 and 6.20×1020 s–1, respectively, meanwhile the apparent activation acquired by Flynn-Wall-Ozawa equation is 210.39 k J·mol–1 The G(a)=(1–α)–1–1(n=2) is regarded as the most appropriate thermal decomposition kinetics equation.In order to understand the influences of technological conditions to the principal products and by-products in depth, excavate the further process potentials, seek a new technological breakthrough, we must do the research of reaction mechanism deeply and master the nitrolysis process essentially... |
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