Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

On the elementary steps of acid zeolite catalyzed amination of light alcohols

Abstract: Potential elementary reaction steps in solid acid catalyzed amination of light alcohols are critically compared using hydrogen mordenite as (model) catalyst and the open mechanistic questions have been addressed. Transient kinetic experiments combined with in situ infrared spectroscopy and isotopic labeling techniques were the main experimental means to provide answers to these questions. While we have shown before that the ammonium ions rapidly react with methanol to produce methylammonium ions, these results show that also methoxy groups formed from methanol at strong Bronsted acid sites under reaction conditions are very low, it is concluded that the contribution of the methoxy group amination to the amine formation is very small. The rate determining step for the overall reaction is the ammonia mediated removal of the alkylammonium ions from the zeolite. It is shown that two interconnected reaction steps govern this, i.e., alkyl group scavenging by ammonia/amines and ammonia/amine adsorption assisted desorption of alkylammonium ions. The combination of both the steps determines the observed rates and selectivities to the different amines. The formation of alkenes in the synthesis of larger alkylamines over strongly acidic catalysts is shown to largely result from Hofmann elimination (decomposition of ethylamines) and not from elimination reactions of the alcohol reactant.

Neural Network Based in Silico Simulation of Combustion Reactions.

Abstract: Understanding and prediction of the chemical reactions are fundamental demanding in the study of many complex chemical systems. Reactive molecular dynamics (MD) simulation has been widely used for this purpose as it can offer atomic details and can help us better interpret chemical reaction mechanisms. In this study, two reference datasets were constructed and corresponding neural network (NN) potentials were trained based on them. For given large-scale reaction systems, the NN potentials can predict the potential energy and atomic forces of DFT precision, while it is orders of magnitude faster than the conventional DFT calculation. With these two models, reactive MD simulations were performed to explore the combustion mechanisms of hydrogen and methane. Benefit from the high efficiency of the NN model, nanosecond MD trajectories for large-scale systems containing hundreds of atoms were produced and detailed combustion mechanism was obtained. Through further development, the algorithms in this study can be used to explore and discovery reaction mechanisms of many complex reaction systems, such as combustion, synthesis, and heterogeneous catalysis without any predefined reaction coordinates and elementary reaction steps.

Studies of the decomposition of oxirane and of its addition to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Abstract: Elementary reactions involved in the oxidation and decomposition of oxirane have been studied in two ways at 480 °C. First, by adding small amounts of oxirane to slowly reacting mixtures of H2+O2, rate constants have been determined for H and OH attack on the additive: H + C2H4O → C2H3O + H2(22H) OH + C2H4O → C2H3O + H2O. (21H) Combination with independent low-temperature data gives values of log10(A22H/dm3 mol–1 s–1)= 10.9±0.2, E22H= 41±2 kJ mol–1 and log10(A21H/dm3 mol–1 s–1)= 10.25±0.15, E21H= 15.1 ± 1.9 kJ mol–1. The major products are CH4 and CO, and CO, and a value of (7.5 ± 2)× 105 dm3 mol–1 s–1 is obtained for the rate constant of the CH3+H2 reaction. Secondly, decomposition studies have been carried out at total pressures (with N2) of 60 and 500 Torr with oxirane pressures varied between 1 and 20 Torr. The major products are CH4, CO, CH3CHO, C2H6, C2H4 and H2, and a comprehensive mechanism is presented to account for their occurrence. In addition to consumption in free-radical processes through H and CH3 attack, oxirane isomerises to give excited CH3CHO* molecules which undergo three competing reactions: [graphic omitted] Little effect of total pressure is observed on the relative rate of each process. From the relative yields of CH4 and C2H6, a value of k4=(4.0 ± 0.2)× 105 dm3 mol–1 s–1 is obtained, which in combination with low-temperature data gives log10(A4/dm3 mol–1 s–1)= 9.03±0.15 and E4= 49.5±1.3 kJ mol–1. It is suggested that C2H4 is formed in radical–radical reactions of the CH2CHO radical. Evidence is presented to support the view that the C—H bond dissociation energy in oxirane is very similar to that in C2H6, and values of D([graphic omitted])= 420.2±4.0 kJ mol–1 and ΔfHo298(C2H3O)= 149.6 kJ mol–1 are suggested: CH3+ C2H4O → CH4+ C2H3O. (4)

Directions of theoretical and experimental investigations into the mechanisms of heterogeneous catalysis

Abstract: The roles of the atomic structure and the electronic structure of the active surface sites in bonding of reactants and causing bond breaking or bond formation have been the focus of theoretical studies. In addition to calculations on static systems, usually clusters, modelling of the transition states and the dynamics of elementary reaction steps (adsorption, dissociation, surface diffusion, desorption) have been performed. Variations of electronic structure of elements across the periodic table have been shown to be responsible for the unique importance of transition metals in catalysis. Experimental studies utilize catalysts with well-characterized structure (zeolites, crystal surfaces) and information about surface structure, composition and chemical bonding of adsorbates becomes available on the molecular level. Deliberate alteration of catalyst structure, surface composition by alloying and electronic structure by addition of electron donor and electron acceptor promoters have been utilized to modify reaction rates and selectivity. This way many of the molecular ingredients of heterogeneous catalytic reactions have been identified. In recent years evidence has been accumulating that indicates periodic and long term restructuring of the catalyst surface as necessary for chemical change and reaction turnover. These findings point to the need of time resolved studies and in-situ investigations of both the substrate and the adsorbate sides of the surface chemical bonds simultaneously on a time scale shorter than the reaction turnover frequency. Close collaboration between theorists and experimentalists is essential if we are to succeed in designing heterogeneous catalysts.