Comparative investigation on antioxidant properties of phenolic compounds in lubricants

Antioxidant properties of several phenolic compounds in lubricants were investigated by quantum mechanical simulation. The reaction barrier of phenol, 2-tert-butylphenol and 2,6-di-tert-butylphenol with the alkoxy free radical generated by lubricants oxidation, the bond dissociation energy of O-H in them, and the energy difference from the HOMO to the LUMO of them were calculated. The reaction activity of phenoxy free radicals formed after hydrogen supply reaction was compared. The results showed that the hydrogen supply capacity of phenolic compounds decreased, while the reaction energy barrier of phenoxy free radicals with lubricants increased obviously with the increase of substituents on benzene ring. As the number of ortho substituents rise, the bond dissociation energy of O-H in phenolic compounds and the energy difference from the HOMO to the LUMO of them decreased slowly. The antioxidant properties of phenolic compounds in lubricants was related not only to the O-H bond dissociation energy and the energy difference from the HOMO to the LUMO of them, but also to steric hindrance in phenolic antioxidants.