Mechanistic Modeling of the Partial Oxidation of 1,3-Propanediol: Comparison of Free-Radical and Concerted Mechanisms

(Graph Presented) The homogeneous partial oxidation of 1,3-propanediol using oxygen at relatively mild conditions (430 K) has been observed to result in high selectivity to acrolein at low to moderate conversion with minor formation of formaldehyde and acetaldehyde (Diaz, E. et al. ChemSusChem 2010, 3, 1063-1070, DOI: 10.1002/cssc.201000142). In this work, various competing mechanistic postulates for the selective conversion of 1,3-propanediol to acrolein were tested quantitatively using mechanistic modeling. Specifically, mechanisms based on free-radical chemistry and concerted pathways, both of which have been postulated in the literature, were evaluated. Automated mechanism generation was used to comprehensively create detailed reaction mechanisms based on reaction families relevant to free-radical or concerted chemistries. A mechanism based on free-radical reactions alone was unable to account for the rate of conversion and high selectivity to acrolein that were observed experimentally. Concerted pathways based on both unimolecular and bimolecular dehydrogenation and dehydration routes successfully rationalized the experimental data, but the activation energies for dehydrogenation reactions required to match the data were significantly lower than those reported theoretically. A model involving both free-radical and concerted chemistry provided the most plausible quantitative description of the experimental data, capturing 1,3-propanediol conversion, product yields, and the dependence of rates on oxygen concentration well.