Generally, puissant characteristic odor emanating from the drying of oil-based paints or other various bio-based coatings creates an undesirable working environment in modern industrial setups. Moreover, in foods comprised of natural oils, such volatile aldehydes have been seen to sour the taste. At present, the composition of these volatiles has been established;, however, the mechanism by which they are produced is yet to be unearthed. Fortunately, microkinetic modelling has emerged as a powerful tool that can offer means to facilitate the exploration of the mechanisms that govern the production of volatile molecules and lead to a novel mechanistic understanding. The application of this technique has led to further discoveries in relation to the significance of the kinetic-routes for the formation of hexanal and pentanal, the main volatile products responsible for the characteristic odor. Unfortunately, the mechanism responsible for the generation of the small volatile aldehydes during low temperature condensed phase oxidation has yet to be explored quantitatively in the context of a kinetic model.
Recently, a team of researchers from the Departments of Chemical & Biological Engineering and Materials Science & Engineering, Northwestern University,: Ms Lindsay Oakley, Professor Kenneth Shull, and Professor Linda Broadbelt, in collaboration with Dr. Francesca Casadio from the Art Institute of Chicago, examined in detail several alternative mechanistic postulates to rigorously test in the context of a previously developed microkinetic model. In such a context, they proposed to parametrize mechanistic postulates within the bounds of theory structures and test them for kinetic relevance within the range of expected computational error. Their work is currently published in the journal, Industrial & Engineering Chemistry Research.
The research team commenced their studies by assembling a variety of mechanistic postulates for the formation of the volatile species hexanal, such as Korcek-like decomposition reactions, intramolecular reactions of allylic peroxy species, direct β-scission of alkoxy radicals, intramolecular hydrogen shifts and scission reactions of higher order oligomeric species. Next, they performed quantum chemical calculations so as to obtain estimates of kinetic parameters necessary to test each reaction’s kinetic relevance in a microkinetic model for the oxidation of cobalt- catalyzed ethyl linoleate.
The authors observed that under atmospheric conditions, the dimeric species was seen to be the largest contributor of hexanal as well as responsible for ending the scission of chain with an induction time evident. Additionally, the research team obtained moreadditional detailed empirical data with information about the early (<24 hr) time window by gas chromatography/mass spectrometry headspace analysis. Their results showed a significant improvement and good agreement between the model and the experimental data when the mechanism involving decomposition of dimeric species was included.
The Oakley and colleagues study successfully explored numerous mechanisms for the formation of volatile products, hexanal in particular, through the use of computational approaches. While the authors often remark that their research involves “watching paint dry”, their computational examination of mechanisms underlying the changes that paints undergo when exposed to air has exciting implications. It has been seen that most of the tested pathways in their work, are also appropriate for oxidation chemistry performed at conditions more extreme than room temperature. Furthermore, the results have indicated that a reasonable mechanism for the formation of volatile products at room temperature can be accomplished by the scission of chain ends from higher order species. Altogether, the mechanism presented by the authors has crucial implications for the formation of an overall cure coating, and the new and improved data can be extended for use to other systems involving oxidation chemistry.
Lindsay H. Oakley, Francesca Casadio, Kenneth R. Shull, and Linda J. Broadbelt. Examination of Mechanisms for Formation of Volatile Aldehydes from Oxidation of Oil-Based Systems. Ind. Eng. Chem. Res. 2018, 57, 139-149Go To Ind. Eng. Chem. Res.