Microkinetic Modeling of Homogeneous and Catalyzed Oxidation Systems

Robert R.O  Brydon

doi:https://doi.org/10.21985/n2-csa7-bz14

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Microkinetic Modeling of Homogeneous and Catalyzed Oxidation Systems

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Oxidation processes play an integral role in many industries, but questions still remain regarding the mechanisms of carbon-hydrogen bond activation and oxidation in free-radical and catalyzed processes. Microkinetic modeling is a powerful tool capable of examining oxidation processes at the mechanistic level to gain understanding and guide catalyst applications. This thesis developed microkinetic models for oxidation of organic substrates using novel and industrially-relevant catalysts. Understanding the role of free-radical and catalyzed oxidation within a system is critical to knowing the effect of catalysts and predicting behavior when adapted to a new process. In this work, microkinetic models examined the oxidation of alcohols, alkenes, and alkylbenzenes using free radicals and various catalysts.', 'Microkinetic modeling was applied to the oxidation of 1,3-propanediol in order to explain the atypically high selectivity to acrolein for an uncatalyzed process. The developed free-radical model did not exhibit enough activity to explain the observed experimental conversion. An initial concerted dehydration reaction is the most probable mechanism of increasing the overall rate of reaction. The dehydration reaction leaves behind a double bond, and the allylic hydrogen is more likely to be initiated by free radicals. The investigation of this system showed that for oxidized starting materials concerted reactions may play an important role in directing selectivity.', 'The microkinetic model was then expanded to the oxidation of cyclooctene catalyzed by gold nanoparticles. There has been significant interest in determining the role of small gold nanoparticles in catalyzing reactions. In order to reveal the role of ~5-8 atom gold clusters, the free-radical model was first validated against experimental data. Net rate analysis revealed that alkoxy and peroxy radicals prefer to add to cyclooctene as opposed to abstract hydrogen, giving rise to high (~80 %) selectivity to cyclooctene oxide. With the underlying chemistry defined, the role of gold nanoparticles was initiation of free radicals via hydrogen abstraction by superoxo gold species and hydroperoxide decomposition on gold sites. This makes gold nanoparticles interesting radical initiators, but they are not a plausible method for guiding selectivity of a system to epoxide products.', 'The established cyclooctene model was amended to account for the addition of another substrate, ethylbenzene, and a new catalyst, Co/ZSM-5. This model provided the opportunity to investigate the effect of kinetically coupling of the slower oxidation of ethylbenzene with the active oxidation of cyclooctene. Net flux analysis revealed that under conditions that have a high concentration of ethylbenzene, alkoxy and peroxy radicals derived from cyclooctene have a higher tendency to abstract hydrogen, initiating ethylbenzene oxidation. The alkoxy and peroxy radicals are more important in initiating ethylbenzene and driving secondary reactions to acetophenone than the zeolite catalyst, showing that kinetic coupling is a viable method of improving activity.', 'Finally, direct epoxidation reactions catalyzed by Ti/SiO2 were added to the extant free-radical model for studying oxidation of limonene. The complexity of limonene led to a far more complex free-radical behavior with diepoxide and 1,2-epoxide products being slightly preferred to a multitude of other minor products. For catalyzed oxidation, the system is highly selective to 1,2-limonene oxide, implying that the 1,2-double bond is the most vulnerable for oxidation in both cases. Examination of the mechanism revealed that the Ti/SiO2 catalyst can be in multiple active states with differing activities, emphasizing the importance of catalyst structure on oxidation.', 'The microkinetic modeling framework established in this work can serve as a platform to investigate a variety of oxidation systems. When built upon, the effect of catalysts can be quantified, and predictions can be made in order to improve oxidation processes.

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