Mechanism and Kinetics for the Initial Steps of Pyrolysis and Combustion of 1,6-Dicyclopropane-2,4-hexyne from ReaxFF Reactive Dynamics

Abstract

We report the kinetic analysis and mechanism for the initial steps of pyrolysis and combustion of a new fuel material, 1,6-dicyclopropane-2,4-hexyne, that has enormous heats of pyrolysis and combustion, making it a potential high-energy fuel or fuel additive. These studies employ the ReaxFF force field for reactive dynamics (RD) simulations of both pyrolysis and combustion processes for both unimolecular and multimolecular systems. We find that both pyrolysis and combustion initiate from unimolecular reactions, with entropy-driven reactions being most important in both processes. Pyrolysis initiates with extrusion of an ethylene molecule from the fuel molecule and is followed quickly by isomerization of the fuel molecule, which induces additional radicals that accelerate the pyrolysis process. In the combustion process, we find three distinct mechanisms for the O(2) attack on the fuel molecule: (1) attack on the cyclopropane, ring expanding to form the cyclic peroxide which then decomposes; (2) attack onto the central single bond of the diyne which then fissions to form two C(5)H(5)O radicals; (3) attack on the alkyne-cyclopropane moiety to form a seven-membered ring peroxide which then decomposes. Each of these unimolecular combustion processes releases energy that induces additional radicals to accelerate the combustion process. Here oxygen has major effects both as the radical acceptor and as the radical producer. We extract both the effective activation energy and the effective pre-exponential factor by kinetic analysis of pyrolysis and combustion from these ReaxFF simulations. The low value of the derived effective activation energy (26.18 kcal/mol for pyrolysis and 16.40 kcal/mol for combustion) reveals the high activity of this fuel molecule.

Enhancing the Oxidation of Toluene with External Electric Fields: a Reactive Molecular Dynamics Study

“…For each simulation, the dynamic trajectories of toluene oxidation, which include atomic positions and velocities, were saved for every 0.1 ps. The bond-order cut off values for each pair of elements used for molecular recognition about the intermediates and products formed during the ReaxFF simulations can be checked elsewhere 25, 43, 44 . The LAMMPS software package was used to perform all the simulations.…”

High-temperature decomposition of the cellulose molecule: a stochastic molecular dynamics study

“…The parameterisation by Chenoweth et al (2008), originally developed for the high-temperature gas-phase oxidation of hydrocarbons, was adopted for this work. It has been used in various studies for simulating the thermal decomposition of hydrocarbon and carbohydrate compounds (Chenoweth et al 2009;Jiang et al 2009;Desai et al 2011;Salmon et al 2009a, b;Wang et al 2011;Liu et al 2011a;Cheng et al 2012). The uncertainties related to the ReaxFF approach in general, and the parameterisation by Chenoweth et al in particular, are acknowledged.…”

Comparison of thermal and catalytic cracking of 1-heptene from ReaxFF reactive molecular dynamics simulations

“…Since RFF parameters are often optimized against QM data and can simulate reaction pathways without any preconditions, reactive MD simulations via RFF schemes represent a useful tool for investigating highly complex reactive systems. In particular, the ReaxFF reactive force field has been used in various studies to examine initial reaction mechanisms and kinetics associated with thermal and catalytic decomposition of several materials [10][11][12][13][14][15].…”