Investigation of the Reaction Path from UO2 to UF6 using Density-functional Theory

A detailed investigation of the reaction mechanism for UO2 reacting with F2 to form UF6 is performed by using density functional theory (DFT). We divide the whole reaction chain into four main steps, UO2+F2→UO2F2, UO2F2+F2→UO2F4, UO2F4→UF4+O2and UF4+F2→UF6. Contrary to what has been mostly expected that F2 molecule should directly replace two O atoms in UO2 molecule, two F2atoms actually combine with UO2 successively in the first two steps. The third step is relatively complex and one F atom in UO2F4 molecule plays a key role of carrying an O atomclose to the other O atom. In last step, F-F bond in F2 molecule fractures and two F atoms are bonding with U atom in UF4 molecule successively. Spin-flip appears in two elementary reactions owing to the existence of heavy atom U.

Comparative study of Metallocene catalyst propylene polymerization with different iteration rates

This paper proposed a mathematical model corresponding to metallocene catalyzed propylene polymerization that uses the Me2Si [Ind]2ZrCL2 and Et [Ind]2ZrCL2. Comprehensive kinetic models consisting of mass and population balance equations, are developed based on elementary reactions proposed in the reaction mechanism. The result from the above indicates that metallocene catalysts in the presence of ethylene zirconium dichloride and methylene zirconium dichloride shows modularity and new peaks are obtained. The temperature variation from 25 to 75 also increase the rate and reason for the same could be chain shuttling polymerization. The model is presented through simulative study. Initially genetic approach is used but convergence rate is poor. To achieve best possible result, particle swarm optimization is used. The optimization approach with particle swarm optimization is implemented. The local and global solutions are comparable entities and replace each other in case value of local variable is not optimized. From the simulative study it is discovered that Et [Ind]2ZrCL2 produce best possible polymers both at 25 and 750C.

Simulation analysis of biomass pyrolysis based on the improved CPD model with chain reaction dynamics

The complex composition and molecular structure of biomass lead to more complex and diversified chemical reactions in the pyrolysis. According to the structural characteristics of the reactants, this paper simplifies the pyrolysis process and extends the research focus from the micro-molecular elementary reactions to the macro reaction kinetics. The wheat straw is chosen as the investigated biomass, and the promoted chemical percolation devolatilization (CPD) with modified pseudo-grid and chain reaction kinetics (CRK) pyrolysis models were constructed for predicting the pyrolysis characteristics. Compared with the experimental results, the prediction errors of char, oil and gas production are in a reasonable range of < 10 %. Moreover, the reliability of the model is verified by comparing with the experimental thermogravimetric curve, which shows that the model could well predict the mass loss, product distribution and component characteristics, and provides a reasonable prediction for the pyrolysis of biomass.

Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes

Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site MnIII-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site MnIII-hydroxo complex is regenerated by the reaction of a MnIII-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of MnIII-hydroxo and MnIII-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (6Medpaq). In this present work, we describe the reaction of the MnIII-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported MnIII-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak MnIII-N(pyridine) bonds lead to a higher MnIII/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the MnIII-alkylperoxo complex [MnIII(OOtBu)(6Medpaq)]+ reacts with water to obtain the corresponding MnIII-hydroxo species, thus mimicking the ligand substitution step proposed for MnLOX.

Universally valid reduction of multiscale stochastic biochemical systems using simple non-elementary propensities

Biochemical systems consist of numerous elementary reactions governed by the law of mass action. However, experimentally characterizing all the elementary reactions is nearly impossible. Thus, over a century, their deterministic models that typically contain rapid reversible bindings have been simplified with non-elementary reaction functions (e.g., Michaelis-Menten and Morrison equations). Although the non-elementary reaction functions are derived by applying the quasi-steady-state approximation (QSSA) to deterministic systems, they have also been widely used to derive propensities for stochastic simulations due to computational efficiency and simplicity. However, the validity condition for this heuristic approach has not been identified even for the reversible binding between molecules, such as protein-DNA, enzyme-substrate, and receptor-ligand, which is the basis for living cells. Here, we find that the non-elementary propensities based on the deterministic total QSSA can accurately capture the stochastic dynamics of the reversible binding in general. However, serious errors occur when reactant molecules with similar levels tightly bind, unlike deterministic systems. In that case, the non-elementary propensities distort the stochastic dynamics of a bistable switch in the cell cycle and an oscillator in the circadian clock. Accordingly, we derive alternative non-elementary propensities with the stochastic low-state QSSA, developed in this study. This provides a universally valid framework for simplifying multiscale stochastic biochemical systems with rapid reversible bindings, critical for efficient stochastic simulations of cell signaling and gene regulation. To facilitate the framework, we provide a user-friendly open-source computational package, ASSISTER, that automatically performs the present framework.

An experimental/computational study of steric hindrance effects on CO2 absorption mechanism using nonaqueous/aqueous AMP

The reaction kinetics and molecular mechanisms of CO2 absorption using nonaqueous and aqueous amine solutions were analyzed by the stopped-flow technique and ab initio molecular dynamics (AIMD) simulations. Pseudo first-order rate constants (k0) of reactions between CO2 and amines were measured. A kinetic model was proposed to correlate the k0 to the amine concentration, and was proved to perform well for predicting the relationship between k0 and the amine concentration. The experimental results showed that AMP/MDEA only took part in the deprotonation of MEA-zwitterion in nonaqueous MEA+AMP/MEA+MDEA. In aqueous solutions, AMP can also react with CO2 through base-catalyzed hydration mechanism beside the zwitterion mechanism. The molecular mechanisms of CO2 absorption were also explored by AIMD simulations coupled with metadynamics sampling. The predicted free-energy barriers of key elementary reactions verified the kinetic model and demonstrated the different molecular mechanisms for the reaction between CO2 and AMP in nonaqueous and aqueous systems.

Galvano-Fenton Engineering Solution with Spontaneous Catalyst’s Generation from Waste: Experimental Efficiency, Parametric Analysis and Modeling Interpretation Applied to a Clean Technology for Dyes Degradation in Water

In this paper, the degradation of the diazo dye naphthol blue black (NBB) using the Galvano-Fenton process is studied experimentally and numerically. The simulations are carried out based on the anodic, cathodic, and 34 elementary reactions evolving in the electrolyte, in addition to the oxidative attack of NBB by HO• at a constant rate of 3.35×107 mol−1·m3·s−1 during the initiation stage of the chain reactions. The selection of the operating conditions including the pH of the electrolyte, the stirring speed, and the electrodes disposition is performed by assessing the kinetics of NBB degradation; these parameters are set to 3, 350 rpm and a parallel disposition with a 3 cm inter-electrode distance, respectively. The kinetics of Fe(III) in the electrolyte were monitored using the principles of Fricke dosimetry and simulated numerically. The model showed more than a 96% correlation with the experimental results in both the blank test and the presence of the dye. The effects of H2O2 and NBB concentrations on the degradation of the dye were examined jointly with the evolution of the simulated H2O2, Fe2+, and HO• concentrations in the electrolyte. The model demonstrated a good correlation with the experimental results in terms of the initial degradation rates, with correlation coefficients exceeding 98%.

Stimulation Calculation of Desulfurization Mechanisms Dominated by Free Radicals Reactions During Pyrolysis of Thiophenes Under Water Vapor Atmosphere

The desulfurization mechanisms of thiophene and 2-methyl thiophene were investigated by the density functional theory (DFT) during pyrolysis under water vapor atmosphere. All possible reaction pathways of these desulfurization mechanisms were explored at M06-2X/6-311g (d) level. The Multwfn3.0 and VMD1.9.2 programs were used to analyze weak interactions between thiophene compounds and H2O molecule. It can be seen that hydrogen bonds can be formed in the reactions of thiophene sulfurs and H2O. Since H2O molecule can decompose at higher temperature and generate free radicals, such as·H and·OH,, the desulfurization mechanisms of thiophene and 2-methyl thiophene with free radicals need to be further considered. The reaction energy barriers (∆G≠) and reaction energies (∆GP) of thiophene and 2-methyl thiophene with H2O molecule (g) or free radicals (·H and·OH) have been stimulated and calculated in detail. Based on the transition state theory (TST), the rate constants corresponding to these elementary reactions are also calculated, meanwhile the speed and spontaneity of every reaction can be obtained from the aspect of kinetics. Theoretically, it is found that H2O (g) directly attacking C-S bonds of thiophene and 2-methyl thiophene cannot easily generate COS and H2S even at 1200 K in terms of thermodynamics and kinetics. If the desulfurization mechanisms of thiophenes are investigated by free radicals mechanisms under steam atmosphere, their initial energy barriers needing to be overcome significantly reduce. Therefore, desulfurization mechanisms of thiophenes and H2O (g) are the most possibly dominated by radical reactions at higher temperatures and H2S is mainly generated.