On the Origin of Formamides in Diarylamine-Inhibited Autoxidations & The Development of Latent Radical Trapping Antioxidants for High-Temperature Applications

Abstract

Autoxidation is a free radical chain reaction by which hydrocarbons undergo oxidative degradation. This process involves insertion of molecular oxygen into hydrocarbon-based materials such as oils, lubricants, polymers, etc. thereby altering their properties. Unlike living organisms, petroleum-derived products do not have inherent mechanisms to combat autoxidation. Therefore, they must be formulated with radical-trapping antioxidants (RTAs) which trap chain-propagating peroxyl radicals, terminating the radical chain. Although this technology has existed for decades, improvements are sought to keep up with ever more stringent environmental regulations, which requires internal combustion engines to operate at higher temperatures than previously, putting the lubricants under far greater oxidative stress. To improve RTAs, the process by which they are consumed and/or deactivated must be further understood. Previous studies in our group revealed that N-formyl alkylated diphenylamines (ADPAs) are predominant products formed during ADPA-inhibited autoxidations of hydrocarbons at elevated temperatures. From previously known autoxidation products (acids, aldehydes, ketones, peracids, and alcohols), we hypothesized that N-formyl ADPAs arise from reactions of ADPAs with formate esters formed from Baeyer-Villiger oxidations of aldehydes. Herein we demonstrate that N-formyl ADPAs can be formed by this pathway. This was followed by a computational study to provide additional insight on the competition between formate ester and carboxylic acid formation, as the latter is almost exclusively observed in B-V oxidations of aldehydes. Surprisingly, the selectivity arising from these competing pathways has never been systematically evaluated as a function of temperature. Additionally, to improve the stability - and therefore efficacy - of highly reactive diarylamine RTAs such as phenothiazine (PTZ) and phenoxazine (PNX), we have investigated the potential of N-acyl PTX and N-acyl PNZ derivatives as latent RTAs. Thus, we prepared various N-acyl derivatives that could serve as precursors to the reactive free amine by either intermolecular or intramolecular acyl substitution in situ. Additionally, we investigated the impact of para-substitution (relative to the amine) to investigate how electronics influence both the rate of release and the efficacy of the RTAs, which were determined in n-hexadecane autoxidations at 165 °C. Combining the performance metrics with kinetics of release will pave the way forward in RTA design.