Experimental And Theoretical Studies On The Thermal Stability Of Nitrates

In this paper, both quantum chemical calculations and experimental methodshave been comprehensively carried out on thermal stability of five nitrates: n-propylnitrate (NPN), isopropyl nitrate (IPN), 2-ethylhexyl nitrate (EHN), tri-ethyleneglycol dinitrate (Tri-EGDN), and tetraethylene glycol dinitrate (Tetra-TEGDN). Thequantum chemical methods such as ab initio, density functional theory (DFT) andseveral semiempirical MO methods were employed to study them and theircorresponding radicals' equilibrium geometries, electronic structures, infraredvibrational spectra, heats of formation and the other thermodynamic properties,pyrolysis mechanism. Thermal decomposition properties of some nitrates were alsoinvestigated with Ambient-Pressure DSC, High-Pressure DSC, and AcceleratingRate Calorimetry (ARC). The main contents were as follows:The DFT-(U) B3P86/6-31+G^* method was employed to investigate theequilibrium geometries of five nitrates and their corresponding radicals. Thevariational rules of geometrical parameters in molecules and radicals were analyzed.Based on the optimized equilibrium geometry, the electronic structures such ascharge distribution, Mulliken population, frontier orbital energy (E_HOMO, E_LUMO) andenergy gap (â–³E=E_LUMO-E_HOMO) as well as dipole moments were also analyzed.The IR frequencies, and thermodynamic properties (H°_m, C°_{p, m} and S°_m) in thetemperature range 298～800 K were calculated for five nitrates and theircorresponding radicals, using the DFT method at the B3LYP/6-31 G^* level.The heats of formation (HOFs) were calculated for five nitrates by usingHartree-Fock and Density Functional Theory (B3LYP and B3P86 methods), with6-31 G^* and 6-31+G^* basis sets via isodesmic reactions, semiempirical MO methodsas well as two additivity schemes. Compared with experimental values available,HOFs from B3P86/6-31 G^* level can be referred to as criteria for five nitrates. On thebasis of our calculations, we recommend HOF values for NPN, IPN, EHN,Tri-EGDN and Tetra-EGDN, are-169.52 kJ·mol^-1, -184.08 kJ·mol^-1, -240.36kJ·mol^-1, -576.88 kJ·mol^-1 and -793.91 kJ·morl^-1, respectively.Under B3LYP/6-31G^*, B3LYP/6-31+G^*, B3P86/6-31G^*, B3P86/6-31+G^*,HF/6-31G^*, and HF/6-31+G^* levels, the bond dissociation energies (BDEs) of O-NO₂ bond in five nitrates were calculated by using the thermo-chemical schemesupplied by Gaussian and Morokuma methods. The pyrolysis mechanism of five titlecompounds is also elucidated.The thermal decomposition characteristics of NPN, IPN, EHN, and Tri-EGDNwere investigated by means of Ambient-Pressure DSC and High-Pressure DSC.Kinetic parameters of thermal decomposition were obtained. The results show thatthe thermal decomposition process of four nitrates under Ambient-Pressure wasoccurred in gas phase. When the pressure increased up to 2MPa, their decompositionprocess was occurred in liquid phase, and the peak temperature of exothermicdecomposition process increased.The thermal stabilities of NPN, IPN, and EHN were investigated using anadiabatic calorimeter called ARC (Accelerating Rate Calorimetry). The curves ofthermal decomposition temperature versus time, self-heating rate versus temperature,pressure versus temperature, and pressure versus self-heating rate were obtained.The thermolysis kinetics and decomposition process under adiabatic condition wereanalyzed, and kinetics parameters such as apparent activation energy,pre-exponential factor and reaction heat were calculated.From the finding that the calculated results of O-NO₂ BDEs in NPN, IPN, andEHN are well coincident with the experimental results of apparent activationenergies from ARC,. we can draw a conclusion that the experimental thermolysis ofthree nitrates is only unimolecular homolytical cleavage of the O-NO₂ bonds.According to the onset temperature (T_onset) and heat of reaction (—△H) of NPN,IPN, and EHN, their levels in hazard classification for thermal reactivity wereobtained. Based on the reactivity risk index (RRI) at 75℃of three nitrates, theirthermal instability were also gained.