Research on the pyrolysis characteristics and mechanisms of waste printed circuit boards at fast and slow heating rates.

The pyrolysis treatment of waste printed circuit boards (WPCBs) shows great potential for sustainable treatment and hazard reduction. In this work, based on thermogravimetry (TG), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and density functional theory (DFT), the thermal weight loss, product distribution, and kinetics of WPCBs pyrolysis were studied by single-step and multi-step pyrolysis at fast (600 °C/min) and slow (10 °C/min) heating rates. The heating rates of TG and Py-GC/MS were the same for each group of experiments. In addition, the bond dissociation energy (BDE) of WPCBs polymer monomers was calculated by DFT method. Compared with slow pyrolysis, the final weight loss of fast pyrolysis is reduced by 0.76 wt%. The kinetic analysis indicates that the activation energies of main pyrolysis stages range from 98.29 kJ/mol to 177.59 kJ/mol. The volatile products of fast pyrolysis are mainly phenols and aromatics. With the increase of multi-step pyrolysis temperature, the order of the escaping volatiles is phenols, hydrocarbyl phenols, aromatics, and benzene (or diphenyl phenol). The pyrolysis residue of WPCBs may contains phenolics and polymers. Based on the free radical reactions, the mechanism and reaction pathways of WPCBs pyrolysis were deduced by the DFT. Moreover, a large amount of benzene is produced by pyrolysis, and its formation mechanism was elaborated.

High-temperature thermal decomposition of iso-octane based on reactive molecular dynamics simulations.

Branched alkanes are the major components of endothermic fuels used for advanced aircrafts. Reactive molecular dynamics (RMD) simulations are carried out to explore the detailed kinetic mechanism for the thermal decomposition of iso-octane widely used as the primary reference fuel of branched alkanes. The RMD calculations indicate that the initial decomposition mechanism of iso-octane is mainly through two pathways: (1) the C - C bond cleavage to produce smaller hydrocarbon radicals and (2) the hydrogen-abstraction reactions by small radicals including •H and •CH3. Most of the alkenes which are associated with the endothermic capacities in the iso-octane pyrolysis are produced from the C-C ß-scission reactions of alkyl radicals. Propylene and ethylene are observed to be formed in large amounts. Kinetic parameters with the activation energy of 52.1 kcal mol-1 and pre-exponential factor of 7.2 × 1014 s-1, based on the first-order kinetic analysis, are in good agreement with previous work.

Initial Thermal Decomposition Mechanism of (NH2)2C=C(NO2)(ONO) Revealed by Double-Hybrid Density Functional Calculations.

This work employs double-hybrid density functionals to re-examine the CO-NO bond dissociation mechanism of nitrite isomer of 1,1-diamino-2,2-dinitro-ethylene (DADNE) into (NH2)2C=C(NO2)O and nitric monoxide (NO). The calculated results confirm that an activated barrier is present in the CO-NO bond dissociation process of (NH2)2C=C(NO2)(ONO). Furthermore, it is proposed that a radical-radical adduct is involved in the exit dissociation path with subsequent dissociation to separate (NH2)2C=C(NO2)O and NO radicals. The activation and reaction enthalpies at 298.15 K for the nitrite isomer dissociation are predicted to be 43.6 and 5.4 kJ mol-1 at the B2PLYP/6-31G(d,p) level, respectively. Employing the B2PLYP/6-31G(d,p) reaction energetics, gradient, Hessian, and geometries, the kinetic model for the CO-NO bond dissociation of (NH2)2C=C(NO2)(ONO) is obtained by a fitting to the modified Arrhenius form 1.05 × 1013(T/300)0.39 exp[-27.80(T + 205.32)/R(T 2 + 205.322)] in units of per second over the temperature range 200-3000 K based on the canonical variational transition-state theory with multidimensional small-curvature tunneling.

Theoretical mechanistic study on the reaction of the methoxymethyl radical with nitrogen dioxide.

Mechanism of the reaction of CH3OCH2 with NO2 is explored theoretically at the M062X/MG3S and G4 levels. The calculated results indicate two stable association intermediates, CH3OCH2NO2 (IM1) and CH3OCH2ONO (IM2), which can be produced by the attack of the nitrogen or oxygen atom of NO2 to terminal carbon atom of CH3OCH2 without barrier involved. IM2 is found to take trans (IM2a)-cis (IM2b) conversion and isomerization to IM1, with the following stability order IM2a > IM2b > IM1. Starting from IM2a, the most feasible pathway is the direct O-NO bond cleavage leading to P1 (CH3OCH2O + NO) or the H-shift and O-NO bond rupture to produce P2 (CH3OCHO + HNO), both of which have comparable contribution to the title reaction. There also involves an H-transfer from the methyl group of IM2a to the N atom with the simultaneous dissociations of the C-O and O-N bonds to produce P4 (2CH2O + HNO). In addition, another dissociation pathway is open to IM2b which decompose to P5 (CH2O + CH3ONO) by the O-N and C-O bond scissions and the recombination of CH3O and NO. Because all the intermediates and transition states involved in the above pathways lie below reactants, the CH3OCH2 + NO2 reaction is expected to be rapid. Subsequent dissociation of IM1 and direct H-abstraction between CH3OCH2 and NO2 are kinetically almost inhibited due to significantly high barriers. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for understanding NO2 combustion chemistry.

Investigation on the Thermal Dissociation of Vinyl Nitrite with a Saddle Point Involved.

Hybrid and double-hybrid density functionals are employed to explore the O-NO bond dissociation mechanism of vinyl nitrite (CH2=CHONO) into vinoxy (CH2=CHO) and nitric monoxide (NO). In contrast to previous investigations, which point out that the O-NO bond dissociation of vinyl nitrite is barrierless, our computational results clearly reveal that a kinetic barrier (first-order saddle point) in the O-NO bond dissociation is involved. Furthermore, a radical-radical adduct is recommended to be present on the dissociation path. The activation and reaction enthalpies at 298.15 K for the vinyl nitrite dissociation are calculated to be 91 and 75 kJ mol-1 at the M062X/MG3S level, respectively, and the calculated reaction enthalpy compares very well with the experimental result of 76.58 kJ mol-1. The M062X/MG3S reaction energetics, gradient, Hessian, and geometries are used to estimate vinyl nitrite dissociation rates based on the multistructural canonical variational transition-state theory including contributions from hindered rotations and multidimensional small-curvature tunneling at temperatures from 200 to 3000 K, and the rate constant results are fitted to the four-parameter Arrhenius expression of 4.2 × 109 (T/300)4.3 exp[-87.5(T - 32.6)/(T 2 + 32.62)] s-1.

Computational Study of the Reaction of Dimethyl Ether with Nitric Oxide. Mechanism and Kinetic Modeling.

To probe into the autoignition effect of nitric oxide (NO) on the combustion of dimethyl ether (DME), a detailed mechanism study and kinetic modeling for the reaction of DME with NO, which was considered to be very sensitive to the ignition delay time of DME, have been conducted using computational chemical methods. The CCSD(T)/6-311+G(2df,2p)//B2PLYP/TZVP compound method was employed to obtain the potential energy surface along the reaction coordinate, with the geometries, gradients, and force constants of nonstationary points calculated at the B2PLYP/TZVP theoretical level. The temperature-dependent rate coefficients from 200 to 3000 K were calculated using multistructural canonical variational transition-state theory (MS-CVT) with torsional motions and multidimensional tunneling effects included. The CCSD(T) calculations with both 6-311+G(2df,2pd) and cc-pVTZ basis sets give a zero-point inclusive barrier of 197-201 kJ mol-1 using the BMK/MG3S, B2PLYP/TZVP, and mPW2PLYP/TZVP based geometries. A van der Waals postreaction complex appears on the products HNO + CH3OCH2 side of the transition state. Two highly coupled torsions lead to four conformers for the transition state, and contributions from multiple structures and torsional anharmonicities substantially affect the rate coefficient evaluations. Variational effects can be argued to play an important role, especially at high temperatures, and tunneling probabilities increase with decreasing temperature. Because of large temperature-dependent feature of activation energy, the four-parameter formula 1.912 × 1011( T/300)3.191 exp[-178.417( T - 2.997)/( T2 + 2.9972)] cm3 mol-1 s-1 is recommended for the MS-CVT calculated rate coefficients including small-curvature tunneling. The kinetic model is shown to give a satisfactory interpretation of the inhibited and accelerated effect of NO on the oxidation of DME.

Variational Effect and Anharmonic Torsion on Kinetic Modeling for Initiation Reaction of Dimethyl Ether Combustion.

The reaction of dimethyl ether (DME) with molecular oxygen has been considered to be the dominant initiation pathway for DME combustion compared to the C-O bond fission. This work presents a detailed mechanism and kinetics investigation for the O2 + DME reaction with theoretical approaches. Using the CCSD(T)/6-311+G(2df,2pd) potential energy surface with the M06-2X/MG3S gradient, Hessian, and geometries, rate constants are evaluated by multistructural canonical variational transition-state theory (MS-CVT) including contributions from hindered rotation and multidimensional tunneling over the temperature range 200-2800 K. The CCSD(T) and QCISD(T) with 6-311+G(2df,2pd) calculations predict a barrier of 190-194 kJ mol-1 for the O2 + DME reaction based on the optimized structures at various levels. It is proposed that there exists a weakly interacting adducts on the product side with subsequent dissociation to the separate HO2 and CH3OCH2 radicals. Torsions in transition state are found to be significantly coupled to generate four conformations whose contributions do influence the rate constant predictions. Variational effects are observed to be significant at high temperatures, while tunneling effect quickly becomes insignificant with temperature. Finally, four-parameter Arrhenius expression 9.14 × 1013(T/300)-3.15 exp[-184.52(T + 110.23)/(T2 + 110.232)] cm3 mol-1 s-1 describes the temperature dependence of MS-CVT rate constants with small-curvature tunneling correction.

Kinetics for the hydrogen-abstraction of CH4 with NO2.

The detailed mechanism of the NO(2)+CH(4) reaction has been computationally investigated at the M06-2X/MG3S, B3LYP/6-311G(2d,d,p), and MP2/6-311+G(2df,p) levels. The direct dynamics calculations were preformed using canonical transition state theory with tunneling correction and scaled generalized normal-mode frequencies including anharmonic torsion. The calculated results indicate that the NO(2)+CH(4) reaction proceeds by three distinct channels simultaneously, leading to the formation of trans-HONO (1a), cis-HONO (1b), and HNO(2) (1c), and each channel involves the formation of intermediate having lower energy than the final product. The anti-Hammond behavior observed in channel 1a is well analyzed. Proper treatment of anharmonic torsions about the C···H···O (or N) axis in the transition structures greatly improves the accuracy of kinetics predictions. The activation energy for each channel increases substantially with temperature, but is not strictly a linear function of temperature. Therefore, the thermal rate constants are fitted to the four-parameter expression recommended for this case over the wide temperature range 400-4000 K.

3,3-Dinitro-azetidinium 2-hy-droxy-benzoate.

In the title gem-dinitro-azetidinium 2-hy-droxy-benzoate salt, C(3)H(6)N(3)O(4) (+)·C(7)H(5)O(3) (-), the azetidine ring is virtually planar, with a mean deviation from the plane of 0.0242 Å. The dihedral angle between the two nitro groups is 87.5 (1)°.

Preparation, non-isothermal decomposition kinetics, heat capacity and adiabatic time-to-explosion of NTOxDNAZ.

NTOxDNAZ was prepared by mixing 3,3-dinitroazetidine (DNAZ) and 3-nitro-1,2,4-triazol-5-one (NTO) in ethanol solution. The thermal behavior of the title compound was studied under a non-isothermal condition by DSC and TG/DTG methods. The kinetic parameters were obtained from analysis of the DSC and TG/DTG curves by Kissinger method, Ozawa method, the differential method and the integral method. The main exothermic decomposition reaction mechanism of NTOxDNAZ is classified as chemical reaction, and the kinetic parameters of the reaction are E(a)=149.68 kJ mol(-1) and A=10(15.81)s(-1). The specific heat capacity of the title compound was determined with continuous C(p) mode of microcalorimeter. The standard mole specific heat capacity of NTOxDNAZ was 352.56 J mol(-1)K(-1) in 298.15K. Using the relationship between C(p) and T and the thermal decomposition parameters, the time of the thermal decomposition from initialization to thermal explosion (adiabatic time-to-explosion) was obtained.

1-(2,4-Dinitro-phen-yl)-3,3-dinitro-azetidine.

In the title compound, C(9)H(7)N(5)O(8), the dihedral angle between the mean plane of the azetidine ring and that of the benzene ring is 26.1 (1)°; the planes of the two nitro groups of the azetidine ring are aligned at 88.7 (1)°.

1-Benzoyl-3,3-dinitro-azetidine.

In the title gem-dinitro-azetidine derivative, C(10)H(9)N(3)O(5), the azetidine ring is almost planar, the maximum value of the endocyclic torsion angle being 0.92 (14)°. The gem-dinitro groups are mutually perpendicular and the dihedral angle between the azetidine and benzene rings is 46.70 (10)°