Initiation Reactions in Acetylene Pyrolysis

Abstract

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.

A modelling study of acetylene oxidation and pyrolysis

“…We would like to note that the increase of k 28 during model optimisation on "the event-related phenomena" is in accordance with the results obtained in [82], what supports our strategy. The finally evaluated rate coefficient value for (R28) is slightly higher than the ∞ value from [82], what follows from the lumping of the CCH 2 radical. The full implementation of the results gained in the [82] will be performed during the model update devoted to the PAH sub-mechanism improvement.…”

“…S3-6, the calculations of the rate coefficients for the association of two acetylene molecules and related reactions performed by Zador et al [82] applying the rigorous ab initio transition-state theory master equation methods lie in the calculated in this study uncertainty intervals. We would like to note that the increase of k 28 during model optimisation on "the event-related phenomena" is in accordance with the results obtained in [82], what supports our strategy. The finally evaluated rate coefficient value for (R28) is slightly higher than the ∞ value from [82], what follows from the lumping of the CCH 2 radical.…”

“…The important "bridge reaction" between small chemistry and the formation of first aromatic ring (R23) was revised. The earlier used experimental data for unimolecular decomposition from Dean [79] was analysed and finally changed to the second order reaction (R38) with k 24 following from Weissman and Benson [80]: Unfortunately, the extensive study of Zador et al [82] came into sight after the final analysis and model validation was done. As studied acetylene mechanism is the part of DLR reaction base, every sub-models included in reaction base are tested after modifications performed for the C1-C2 chemistry.…”

Glassy Carbon: A Promising Material for Micro- and Nanomanufacturing

“…The term pyrolysis is not limited to the heat-treatment of polymers, it defines the thermochemical decomposition of any organic precursor, including gaseous hydrocarbons (e.g., acetylene [ 32 , 37 , 38 ]), hydrocarbon-rich oils [ 39 ] and various petroleum byproducts [ 40 ]. Gas-phase pyrolysis (i.e., thermal cracking of hydrocarbons [ 41 , 42 ]) followed by carbon deposition is generally referred to as the Chemical Vapor Deposition (CVD) of carbonaceous materials in the contemporary literature [ 43 ]. Another well-known application of pyrolysis is in the treatment of waste polymers (both synthetic and natural) for biofuel [ 1 , 44 , 45 ], and biogas [ 46 ] production.…”

Encoding of vinylidene isomerization in its anion photoelectron spectrum

“…T he 1,2-hydrogen shift is the simplest bondbreaking isomerization reaction in organic chemistry (1), and the prototypical example of this process is the isomerization of vinylidene (H 2 CC) to acetylene (HCCH). Vinylidene, the smallest unsaturated carbene (2), has been implicated as a transient intermediate in many chemical processes (3)(4)(5)(6) but is of particular interest as a high-energy form of acetylene (7). From the perspective of chemical physics, the H 2 CC⇌HCCH isomerization (Fig.…”