Resolving Some Paradoxes in the Thermal Decomposition Mechanism of Acetaldehyde

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• 作者：Raghu Sivaramakrishnan ; Joe V. Michael ; Lawrence B. Harding ; Stephen J. Klippenstein
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：July 16, 2015
• 年：2015
• 卷：119
• 期：28
• 页码：7724-7733
• 全文大小：423K
• ISSN：1520-5215

文摘

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 CHb>3b>CHO and its deuterated analogs, CHb>3b>CDO, CDb>3b>CHO, and CDb>3b>CDO. The observations of these products prompted the authors of these studies to suggest that the enol tautomer, CHb>2b>CHOH (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 CHb>3b>CHO potential energy surface (PES). The lowest-energy process on this PES is an isomerization of CHb>3b>CHO to CHb>2b>CHOH. However, the subsequent product channels for CHb>2b>CHOH are substantially higher in energy, and the only unimolecular process that can be thermally accessed is a reisomerization to CHb>3b>CHO. The incorporation of these new theoretical kinetics predictions into models for selected literature experiments on CHb>3b>CHO thermal decomposition confirms our earlier experiment and theory-based conclusions that the dominant decomposition process in CHb>3b>CHO at high temperatures is C鈥揅 bond fission with a minor contribution (鈭?0鈥?0%) from the roaming mechanism to form CHb>4b> and CO. The present modeling efforts also incorporate a master-equation analysis of the H + CHb>2b>CHOH potential energy surface. This bimolecular reaction is the primary mechanism for removal of CHb>2b>CHOH, which can accumulate to minor amounts at high temperatures, T > 1000 K, in most lab-scale experiments that use large initial concentrations of CHb>3b>CHO. Our modeling efforts indicate that the observation of ketene, water, and acetylene in the recent microtubular experiments are primarily due to bimolecular reactions of CHb>3b>CHO and CHb>2b>CHOH with H-atoms and have no bearing on the unimolecular decomposition mechanism of CHb>3b>CHO. 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.