Theoretical Studies On The Properties Of Aromatic Amine And Hindered Phenolic Antioxidants

In practice, the auto-oxidation process is likely to happen for lubricating oil under service conditions since it can be in contact with oxygen easily, leading to the deterioration of oil. Antioxidants can inhibit or retard the auto-oxidation degradation of lubricating oil effectively. Diarylamine and hindered phenol antioxidants are the most widely used ashless antioxidants, and they have received increasing extensive attentions along with the requirements of environmental protection. However, there are some experimental limitations due to the low stability and toxicity of some compounds and difficulties in achieving highly pure samples. Therefore, there is an urgent need to investigate the antioxidantation mechanisms and other properties for antioxidants through quantum chemistry theory at the molecule level, in order to better understand its action mechanism and provide reasonable suggestions for experimental studies. The main results in this paper are as follows:1. Diarylamine (A12NH) is a kind of high-effective antioxidant under the conditions of high temperature, and its high efficiency is one of the hot spot all the time. The catalytic inhibition mechanism was studied using quantum chemistry theory in detail. The results show that the reaction of ROO radical with Ar2NH leads to the activation of Ar2NH, and follows PCET mechanism. It is noteworthy that oxygen may serve as a catalyst to participate in the reaction. Ar2NOR can decompose via β-H mechanism through reacting with ROO radical, resulting in the formation of Ar2N· The resulted Ar2N· can react with an R· radical to regenerate Ar2NH which reenter in the reaction system to complete the catalytic cycle. The4and4’sites substituted ecectron withdrawing groups are unfavorable to the β-H transfer reaction of Ar2NOR with ROO· and then affect the antioxidant cycle disadvantageously.2. The wear and tear of equipments and the use of inorganic additives will bring some metal ions into lubricating oil which may affect the antioxidant performances of antioxidants. Quantum chemistry theory was used to study the influences of coordination effects of K+, Ca2+Mg2+and Fe3+metal ions on the antioxidant properties of diarylamines. The results show that the binding energies between the metal ions and4,4’-dimethyldiphenylamine (DMDPA) are related to the oxidizability of metal ions. The stronger the oxidizability of metal ion is, the smaller the binding energy is. The coordination effect of K+is conducive to reducing the dissociation energy of Ar2N-H bond, however, Ca2+, Mg2+and Fe3+make it larger; Additionally, the coordination effect of K+can effectively reduce the activation energy of the reaction of DMDPA with ROO, and can also make the reaction more favorable in thermodynamics. This suggests that the synergetic antioxidation of alkali metal in lubricating oil containing AX2NH antioxidants could be ascribed to the coordination of the alkali metal ion to the Ar2NH and therefore increasing the reaction rate of capturing ROO radical.3. BHT is a widely used hindered phenol antioxidant which has been proved to existing in the environment due to its high volatility. Degradation reactions of BHT initiated by·OH were investigated detailedly using quantum chemistry theory. The results indicate that the attack of·OH radical on the aromatic ring of BHT leads to the generation of BHT-Q. The attack of·OH radical on the substituted methyl of BHT generates BHT-OH, BHT-CHO and BHT-COOH in turn. And producing BHT-COOH is favorable in thermodynamics. The degradation reactions of BHT initiated by·OH are easy to occur in gaseous and aqueous environment. Therefore, the role of OH radicals is important for the degradation of BHT in environment.