Initiation Reactions in Acetylene Pyrolysis

In
gas-phase combustion systems the interest in acetylene stems
largely from its role in molecular weight growth processes. The consensus
is that above 1500 K acetylene pyrolysis starts mainly with the homolytic
fission of the C–H bond creating an ethynyl radical and an
H atom. However, below ∼1500 K this reaction is too slow to
initiate the chain reaction. It has been hypothesized that instead
of dissociation, self-reaction initiates this process. Nevertheless,
rigorous theoretical or direct experimental evidence is lacking, to
an extent that even the molecular mechanism is debated in the literature.
In this work we use rigorous ab initio transition-state theory master
equation methods to calculate pressure- and temperature-dependent
rate coefficients for the association of two acetylene molecules and
related reactions. We establish the role of vinylidene, the high-energy
isomer of acetylene in this process, compare our results with available
experimental data, and assess the competition between the first-order
and second-order initiation steps. We also show the effect of the
rapid isomerization among the participating wells and highlight the
need for time-scale analysis when phenomenological rate coefficients
are compared to observed time scales in certain experiments.