Mechanistic Modeling of Hydrocarbon Autoxidation: Theory and Application to the Study of Lubricant DegradationPublic Deposited
Autoxidation is a primary route by which lubricants in working conditions undergo degradation. Degraded lubricants, if not replaced periodically, contribute to increased wear and product failure. Experimental studies of lubricant degradation have yielded useful results but have left important questions unanswered. While many computational studies of degradation have been carried out to address these questions, all degradation models which have been proposed thus far fail to agree quantitatively with experimental data and often do not even give qualitative agreement. The purpose of this research is to develop chemical kinetics models of lubricant degradation. Due to the complexity of autoxidation chemistry in the condensed phase, it is prohibitive to assemble a reaction mechanism manually. Therefore, the principles of automated mechanism generation were used to aid in the assembly of reaction networks which model lubricant degradation. Specifically, new reaction networks were established for condensed-phase autoxidation of n-octane and n-decane. The sensitivity of the kinetic parameters was explored and limited parameter estimation was used to improve the agreement between the models and experimental data. Quick and reliable estimation of reaction kinetics is essential since complete experimental characterization of complex reacting systems is not currently possible. However, quantum mechanics and transition state theory can augment our knowledge of chemical reactivity where experiments have failed to provide sufficient information. A computational framework was developed that incorporates electronic structure methods with a rigorous treatment of one-dimensional anharmonic motions to provide accurate thermodynamic and kinetic properties for a wide range of reactions. Using this approach we have developed structure-reactivity relationships for several reaction families important to lubricant degradation: hydrogen transfer, hydrogen abstraction and beta-scission. These new relationships provided chemical and physical insight to the governing reaction families in lubricant degradation and added to a library of structure-reactivity relationships suitable for studying oxidation chemistry. For example, complex contrathermodynamic behavior in the reactions of alkylperoxy radicals and hydrocarbons was uncovered, and a new hypothesis to explain this behavior was given.