Resolving Some Paradoxes in the Thermal Decomposition Mechanism of Acetaldehyde

The mechanism for the thermal decomposition of acetaldehyde has been revisited with an analysis of literature kinetics experiments using theoretical kinetics. The present modeling study was motivated by recent observations, with very sensitive diagnostics, of some unexpected products in high temperature microtubular reactor experiments on the thermal decomposition of CH₃CHO and its deuterated analogs, CH₃CDO, CD₃CHO, and CD₃CDO. The observations of these products prompted the authors of these studies to suggest that the enol tautomer, CH₂CHOH (vinyl alcohol), is a primary intermediate in the thermal decomposition of acetaldehyde. The present modeling efforts on acetaldehyde decomposition incorporate a master equation reanalysis of the CH₃CHO potential energy surface (PES). The lowest-energy process on this PES is an isomerization of CH₃CHO to CH₂CHOH. However, the subsequent product channels for CH₂CHOH are substantially higher in energy, and the only unimolecular process that can be thermally accessed is a reisomerization to CH₃CHO. The incorporation of these new theoretical kinetics predictions into models for selected literature experiments on CH₃CHO thermal decomposition confirms our earlier experiment and theory-based conclusions that the dominant decomposition process in CH₃CHO at high temperatures is C–C bond fission with a minor contribution (∼10–20%) from the roaming mechanism to form CH₄ and CO. The present modeling efforts also incorporate a master-equation analysis of the H + CH₂CHOH potential energy surface. This bimolecular reaction is the primary mechanism for removal of CH₂CHOH, which can accumulate to minor amounts at high temperatures, <i>T</i> > 1000 K, in most lab-scale experiments that use large initial concentrations of CH₃CHO. Our modeling efforts indicate that the observation of ketene, water, and acetylene in the recent microtubular experiments are primarily due to bimolecular reactions of CH₃CHO and CH₂CHOH with H-atoms and have no bearing on the unimolecular decomposition mechanism of CH₃CHO. The present simulations also indicate that experiments using these microtubular reactors when interpreted with the aid of high-level theoretical calculations and kinetics modeling can offer insights into the chemistry of elusive intermediates in the high-temperature pyrolysis of organic molecules