A DFT and kinetic study: Is it possible to prepare epoxides without catalysts using the in-situ generated peroxy radicals or peroxides by one-step method?

DFT calculations and kinetic analysis have been employed to comprehensively explore the possibility to prepare epoxides by one-step method using the in-situ generated peroxy radicals or hydroperoxides as epoxidizing agents. Computational studies demonstrated that the selectivities for the reaction systems of O2 /R2/R1, O2 /CuH/R1, O2 /CuH/styrene, O2 /AcH/R1 were 68.2%, 69.6%, 100% and 93.3%, respectively. The in-situ generated peroxide radicals, such as HOO˙, CuOO˙ and AcOO˙, could react with R1 or styrene by attacking the CC double bond to form a CO bond and subsequently undergoing a cleavage of OO bond to yield epoxides. Peroxide radicals could abstract a hydrogen atom from methyl group on R1, forming unwanted by-products. It should be noted that the hydrogen atoms of HOO˙ is easy to be abstracted by CC double bond and simultaneously the oxygen atom is connected to the CH moiety to form an alkyl peroxy radical (Rad11), greatly limiting the selectivity. The comprehensive mechanistic studies provide a deep understanding on preparing epoxides by one-step method.

Effects of Oxygen: Experimental and VTST/DFT Studies on Cumene Autoxidation with Air under Atmospheric Pressure.

The mechanism of how oxygen affects cumene autoxidation related to temperature is still bewildering. Kinetic analysis of cumene autoxidation with air at a pressure of 1.0 atm was investigated by experiments and variational transition state theory/DFT. Oxygen was the limiting factor for cumene autoxidation above 100 °C, although it had negligible impacts on cumene autoxidation at 70-100 °C. The kinetic analysis by VTST coupled with DFT calculations proved that {k 6,reverse[ROO•]}/{k 7,forward[RH]0 [ROO•]} > 103 (70-120 °C), suggesting that ROO• tended to decompose back to R• and O2 rapidly, whereas it was much slower for ROO• abstracting a hydrogen atom from RH to form ROOH. When the concentration of oxygen was higher than the critical value ([O2]critical), it could not significantly affect the equilibrium concentration of ROO•, which in turn could not affect the autoxidation rate significantly. Besides, the critical oxygen concentration ([O2]critical) was exponentially related to 1/T, which was consistent with Hattori's experimental results.

A stable low-temperature H2-production catalyst by crowding Pt on α-MoC.

The water-gas shift (WGS) reaction is an industrially important source of pure hydrogen (H2) at the expense of carbon monoxide and water1,2. This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures3. Here we demonstrate that the structure (Pt1-Ptn)/α-MoC, where isolated platinum atoms (Pt1) and subnanometre platinum clusters (Ptn) are stabilized on α-molybdenum carbide (α-MoC), catalyses the WGS reaction even at 313 kelvin, with a hydrogen-production pathway involving direct carbon monoxide dissociation identified. We find that it is critical to crowd the α-MoC surface with Pt1 and Ptn species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/α-MoC (ref. 4), and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum. We anticipate that the strategy demonstrated here will be pivotal for the design of highly active and stable catalysts for effective activation of important molecules such as water and carbon monoxide for energy production.

Modified Group Contribution Scheme to Predict the Glass-Transition Temperature of Homopolymers through a Limiting Property Dataset.

Previous studies on glass-transition temperature (T g) prediction mainly focus on developing diverse methods with higher regression accuracy, but very little attention has been paid to the dataset. Generally, a large range of T g values of a specified polymer could be found in the literature but which one should be selected into a dataset merely depends on the implicit preference rather than a recognized and clear criterion. In this paper, limiting glass-transition temperature (T g(∞)), a constant value obtained at the infinite number-average molecular weight M n, was validated to be an adequate bridge index in the T g prediction models. Furthermore, a new dataset containing 198 polymers was established to predict T g(∞) using the improved group contribution method and it showed a good correlation (R 2 = 0.9925, adjusted R 2 = 0.9894). The method could also generate T g-M n curves by introducing the T g(∞) function and provide more information to polymer scientists and engineers for material selection, product design, and synthesis.

Experimental study of thermophysical properties of coal gangue at initial stage of spontaneous combustion.

The specific heat capacity (Cp), thermal conductivity (λ), and thermal diffusion coefficient (D) of coal gangue (CG) are the main factors that affect the self-ignition potential for heat transferring from burning center to the ground surface. In this paper, the thermophysical properties of CG were investigated by transient plane source method. The correlations and sensitivity analysis were performed to characterize the degree of influence of the thermophysical parameters (Cp, λ, and D) dependence on temperature. The mean values of Cp, λ, and D for CG were at a range of 0.73-0.89 J g-1 K-1, 0.44-0.76 W m-1 K-1, and 0.26-0.43 mm2 s-1, respectively. Compared with coal, CG were located in the low area (Sc < 2) with higher value of λ and D, but lower value of Cp. Results also showed that 70 °C was a critical point for CG at which some kinds of mutation took place in thermophysical properties. The comparison between the experimental data and the correlation outputs exhibited consistency.

Inhibition evaluation of gas inhibitors in micron-sized aluminum dust explosion.

The inhibition effects of gas inhibitors (nitrogen, carbon dioxide, and heptafluoropropane) on aluminum dust explosion were investigated experimentally and numerically. The results showed that as the inhibition volume fraction increased, the flame propagation characteristics parameters and explosion severity parameters were inhibited by inert gases accordingly. The inhibition performance of carbon dioxide was superior to that of nitrogen, and the minimum inhibition volume fractions of nitrogen and carbon dioxide were determined. XRD results indicated that the crystal form of major condensed product of aluminum dust explosion using two kinds of inert gas as inhibitors was different due to the distinct inhibition effect. Moreover, the XPS analysis revealed that the nitrogen oxide of aluminum adsorbed on the surface of aluminum particles blocked gasification process of aluminum particles. To explore the inhibition mechanism microscopically, a kinetic model concerning gas phase combustion was established. The above discussion indicated that the inhibition effect was the combination of multiple factors. In addition, due to the strong reactions between aluminum particles and heptafluoropropane, it cannot be regarded as gas inhibitor in aluminum dust explosion.

Suppression mechanism of Al dust explosion by melamine polyphosphate and melamine cyanurate.

The suppression mechanism of melamine polyphosphate (MPP) and melamine cyanurate (MCA) for Al dust explosions is investigated experimentally and computationally. Results show that depending on the concentration of suppressants, the addition of MCA and MPP promotes or suppresses Al dust explosion. For high additive concentration, large agglomerated residues are generated, and condensed phase residues may contain Al particles, MCA or MPP. The chemical composition of condensed phase residues of Al/MCA mixture explosion is mainly Al2O3 and the high boiling products of MEL decomposition (mainly C‒ and N‒containing species). The explosion residues of Al/MPP mixture are composed of Al2O3, high boiling products of MEL decomposition and condensed phosphates. To understand the reasons for pressure enhancement and explosion suppression, a kinetic model considering both gas and surface chemistry of Al particles combustion is developed. The simulations indicate that the high pressure rise is caused by the extra heat released from the exothermic reactions of suppressants and the increase of gas phase products. MPP and MCA can suppress surface reaction by decreasing Al(L) site fraction. Additionally, the vaporization rate of Al particles and the diffusion rate of oxidizers close to the droplet surface are reduced by MPP and MCA addition.

Experimental research on explosion suppression affected by ultrafine water mist containing different additives.

The suppression effects of pure ultrafine water mist and 5% mass fraction alkali metal (NaCl, Na2CO3, KHCO3, KCl and K2CO3) solutions ultrafine water mist on methane explosion were conducted under five mist concentrations in a sealed visual vessel. Mist diameters of different additive solutions were measured by a phase doppler particle analyzer. Pressure data and dynamic flame pictures were recorded respectively by a high-frequency pressure sensor and a high-speed camera. Results indicate that alkali metal compound can enhance the suppression effect of ultrafine water mist and it was related to the additive type. The suppression order of alkali metal compound for methane explosion was K2CO3>KCl > KHCO3>Na2CO3>NaCl. Meanwhile, additive radicals can obviously affect explosion intensity and it mainly reflected in the reduction of explosion pressure under different mist conditions (K+>Na+, Cl- >HCO3-). The pressure generated from combustion wave accelerating propagation underwent two accelerating rises and was affected by additive type and mist amount. The effect of additive type on explosion intensity (maximum explosion overpressure (ΔPmax), two peak values of pressure rising rate) was similar with flame propagation velocity and were decreased evidently with increasing mist concentration. The enhancement in explosion suppression was due to the combination of improved physical and chemical effects.

Flame suppression mechanism of aluminum dust cloud by melamine cyanurate and melamine polyphosphate.

The suppression effect of melamine cyanurate (MCA) and melamine polyphosphate (MPP) on the flame propagation of aluminum dust were studied experimentally. The results indicated that the concentration of MPP required to supress the aluminum dust explosion was lower than MCA. As the concentration of suppressant increased, the acceleration and the maximum flame speed extremely decreased, and the flame morphology became isolated spot flames. The MPP addition exerted a stronger suppression effect on the temperature of aluminum flame compared to MCA addition. Meanwhile, the mechanism of flame suppression was further investigated. Decomposition of MCA and MPP particles could absorb the heat released from the flame front. Scanning electron microscopy demonstrated that the gas phase reaction of aluminum particles was suppressed by MCA and MPP, resulting in a larger particle size of the explosion products. XRD results indicated that MCA and MPP did not react with aluminum. Gaseous products of suppressant decomposition altered flame chemistry by radically recombining O atom and reducing AlO, which resulted in less amount of heat release, lower flame speed and lower flame temperature.

Inhibition evaluation of ABC powder in aluminum dust explosion.

To investigate the inhibition effect of ABC powder for 5 µm and 30 µm aluminum dust explosions, a standard 20 L spherical chamber was employed to determine the explosion severity of Al/ABC mixtures and the MIC (minimum inerting concentration) of ABC powder. Results showed that the MIC increased as the concentration of aluminum dust increased. Meanwhile, the MIC decreased with the declining particle diameter of ABC but increased with the reducing diameter of the aluminum particles. SEEP (suppressant enhanced explosion parameter) phenomenon was observed when the concentration of ABC powder was low. To reveal the enhancement mechanism of ABC powder, explosion severity of NH3 and Al/NH3 mixture was also determined. Results showed that the explosion severity of aluminum dust was significantly enhanced by adding NH3. Scanning electron microscopy showed that the particles of the explosion residues of Al/NH3 mixture and Al/ABC mixture were significantly larger than those of pure aluminum explosion residues. X-ray photoelectron spectroscopy revealed that the major explosion products of Al/NH3 mixture was Al2O3. Explosion products of Al/ABC mixture were mainly comprised of Al2O3 and P2O5. The inhibition mechanism of aluminum dust explosion by ABC powder was summarized systematically based on these results.

Suppression of polymethyl methacrylate dust explosion by ultrafine water mist/additives.

The suppressions of ultrafine water mists containing additives (NaCl and NaHCO3) on 100 nm, 5 µm, and 30 µm polymethyl methacrylate (PMMA) dust explosions were experimentally studied in a dust-explosion apparatus. High-speed photography showed that maximum vertical positions and flame propagation velocities were significantly decreased by suppression with ultrafine water mist/additives. Flame propagation velocities in 100 nm, 5 µm, and 30 µm dust explosions suppressed by the ultrafine pure water mist were reduced by 48.2%, 27.7%, and 15.3%, respectively. Maximum temperatures and temperature rising rates measured by a fine thermocouple in nano- and micro-PMMA dust explosions were also significantly decreased. It was proved that the addition of NaCl and NaHCO3 improved the suppression effects of the ultrafine pure water mist. The improvement of explosion suppression by an 8% NaHCO3 mist was superior to that of a 16% NaCl mist. The suppression mechanisms of ultrafine water mist/additives are further discussed by analyzing the physical and chemical effects.

Inhibition of aluminum dust explosion by NaHCO3 with different particle size distributions.

NaHCO3 with three particle size distributions was employed to determine the minimum inerting concentration (MIC, g/m3) for the explosion of 5µm and 30µm aluminum dust in a standard 20L spherical chamber and thus examine the effect of particle size on the inhibition efficiency. Results showed that the MIC significantly increases as the aluminum particle size decreases from 30µm to 5µm. For 30µm aluminum, the MIC dramatically decreased with the reduction in the NaHCO3 particle size. By contrast, for 5µm aluminum, the MIC was nearly independent of the particle size of NaHCO3 in the range studied. Time-scale analysis indicated that the decomposition of NaHCO3 must be faster than the aluminum combustion reaction for effective chemical inhibition. Scanning electron microscopy showed that the particles of the explosion residues of a NaHCO3/Al mixture were considerably larger than those of pure aluminum explosion residues. A diameter ratio ßmix was defined to evaluate the degree of incomplete reaction promoted by NaHCO3. The composition of the explosion products was analyzed by X-ray photoelectron spectroscopy, and the data revealed that Na2CO3 and Al2O3 are the major species of the products. An inhibition mechanism was proposed based on these results.

Experimental research of heat-mass coupling response of liquid storage tanks.

In order to study the coupling response process of the liquid tank under the external thermal attack, experiments were carried out on a vertical cylindrical storage tank by electric heating. The response processes of the tank pressure, the medium temperatures, and the wet and dry tank wall temperatures were monitored in the experiments. The coupling responding process between the evolving characters of the medium temperature and the tank wall temperature, as well as the resulting rising features of the tank pressure were analyzed comprehensively. The results indicated that, on the one side the heat transfers process across and through the tank wall were influenced obviously by the thermal-flow fields of the two phase mediums which differ a lot in the thermal physical properties. On the other side, affected by the temperature rise in the wall, the vapor medium became thermal stratified and overheated with respect to the tank pressure, and the flow regime in the liquid medium would transform from causing stratification to promoting de-stratification affected by the wall boiling phenomenon.

Experimental research on the characteristics of methane/air explosion affected by ultrafine water mist.

The inhibition effects of ultrafine water mists on 6.5%, 8%, 9.5%, 11%, and 13.5% methane explosions were experimentally studied in a sealed visual vessel. The mist (10µm) produced by a mist generation system in the vessel was measured by a phase doppler particle analyzer. A high-speed camera was used to record the explosion flame affected by spraying concentration and a high frequency pressure sensor was used to acquire the explosion pressure. Meanwhile, the relationship between flame propagation and pressure rising with time was analyzed. The appearance height of "tulip" flame was increased and appearance moment was delayed obviously with the mist amount increased. The variation trend was illustrated from the viewpoint of the interactions among the flame front, the flame-induced reverse flow and the vortices. Moreover, cellular structure appeared in the burned zone and experienced four developing stages, and its formation indicates that water vapor can cause the intrinsic flame instability and absorb heat on the burned zone further. The pressure underwent two accelerating rises, which was affected by mist amount. The accelerating rise processes were related to the accelerating propagation of combustion wave. Furthermore, methane explosion can be absolutely suppressed by the mist.

Suppression of methane/air explosion by ultrafine water mist containing sodium chloride additive.

The suppression effect of ultrafine mists on methane/air explosions with methane concentrations of 6.5%, 8%, 9.5%, 11%, and 13.5% were experimentally studied in a closed visual vessel. Ultrafine water/NaCl solution mist as well as pure water mist was adopted and the droplet sizes of mists were measured by phase doppler particle analyzer (PDPA). A high speed camera was used to record the flame evolution processes. In contrast to pure water mist, the flame propagation speed, the maximum explosion overpressure (ΔP(max)), and the maximum pressure rising rate ((dP/dt)max) decreased significantly, with the "tulip" flame disappearing and the flame getting brighter. The results show that the suppressing effect on methane explosion by ultrafine water/NaCl solution mist is influenced by the mist amount and methane concentration. With the increase of the mist amount, the pressure, and the flame speed both descended significantly. And when the mist amount reached 74.08 g/m(3) and 37.04 g/m(3), the flames of 6.5% and 13.5% methane explosions can be absolutely suppressed, respectively. All of results indicate that addition of NaCl can improve the suppression effect of ultrafine pure water mist on the methane explosions, and the suppression effect is considered due to the combination effect of physical and chemical inhibitions.

Effect of fire engulfment on thermal response of LPG tanks.

A model has been developed to predict the thermal response of liquefied-pressure gases (LPG) tanks under fire, and three-dimensional numerical simulations were carried out on a horizontal LPG tank which was 60% filled. Comparison between numerical predictions and published experimental data shows close agreement. The attention is focused on the influence of different fire conditions (different fire scenarios, various engulfing degrees and flame temperatures) on thermal response of LPG tanks. Potential hazard probabilities under different fire conditions were discussed by analyzing the maximum wall temperature and media energy after the internal pressure rose to the same value. It is found that the less severe fire scenario and lower engulfing case may lead to a greater probability of burst hazard because of the higher maximum wall temperature and media energy before the pressure relief valve (PRV) opens.