Catalytic inhibition of hydrocarbon autoxidation by secondary amines and nitroxides

Abstract

Abstract.A study has been made of the catalytic inhibition of the autoxidation of hydrocarbons at 130°C by secondary amines and derivatives thereof (hydroxylamines and nitroxides). Owing to a better resistance towards side-reactions that cause decomposition of the inhibitor, certain dialkylamines are much more efficient than diarylamines. Some of the dialkylamines examined are capable of scavenging several hundreds of propagating radicals per molecule of inhibitor in autoxidizing alkanes and a few thousand radicals in autoxidizing n-hexylbenzene.

Acid Is Key to the Radical-Trapping Antioxidant Activity of Nitroxides

“…For example, in the pioneering experiments carried out by Bolsman, Blok, and Frijns, 5 which showed a stoichiometric number of 510 for the inhibition of paraffin oil autoxidation, only 1−4 μM TEMPO was used. Given that [O 2 ] ∼ 2 mM, 38 the rate of alkoxyamine formation must be at least 10000 times slower than the rate of peroxyl radical formation.…”

“…5 Thus, under weakly acidic conditions, TEMPO must be regenerated from its corresponding oxoammonium ion, TEMPO + . 13 In sharp contrast, in the presence of strong acids (e.g., trifluoroacetic acid), only one peroxyl radical was trapped, suggesting that TEMPO could not be regenerated from TEMPO + .…”

Multifunctional Antioxidants with High Activity at Elevated Temperatures Based on Intramolecular Synergism

“…The elevated temperatures can facilitate the regeneration of the diphenylamine through retro-carbonylene (RCE) process. [8] However, it must be pointed out that each single moiety can play an efficient role only when their relative position is reasonable. The remarkable activity of SSPDs can be summarized in four points: firstly, the large conjugated system provided by the phenolic Schiff base moiety can greatly stabilize the formed diphenylaminyl radical and facilitate their reaction towards the radicals; secondly, ROOH can be decomposed intramolecularly without escaping from the "cage", so as to reduce the possibility of degrading into more radicals before being captured by the added peroxide decomposers; [20] thirdly, all of the generated sulfur-bearing derivatives have the distinctive capacity to decompose more peroxides; [21] finally, the diphenylamine can be quickly regenerated, resulting in the large stoichiometric efficiency per molecule.…”

“…[5] Numbers of the vitamin derivatives functionalized with nitrogen atoms [6] or chalcogen substituents [7] were successfully prepared and performed excellent reactivity towards radicals under mild conditions. [8] This superior behavior at elevated temperatures enables arylamines preferentially to be incorporated into a range of materials such as rubbers, lubricants, fuels, and plastics. Previous research indicates that the molar ratio of diphenylamine can sharply increase to 41 for the inhibited autoxidation of paraffin oil at 130°C due to its catalytic mechanism (the molar ratio detected at room temperature is only 2).…”

“…Previous research indicates that the molar ratio of diphenylamine can sharply increase to 41 for the inhibited autoxidation of paraffin oil at 130°C due to its catalytic mechanism (the molar ratio detected at room temperature is only 2). [8] This superior behavior at elevated temperatures enables arylamines preferentially to be incorporated into a range of materials such as rubbers, lubricants, fuels, and plastics. In spite of this, the activity of the synthesized compound is always restricted to its instability at high temperature.…”