Interactions of Antioxidants with NOx at Elevated Temperatures

Automotive engines contain aggressive environments where lubricants are degraded by exposure to corrosive combustion by-products such as nitrogen oxides (NO2 and NO), and understanding their interaction is key to the development of long-lived lubricants. Therefore the effect of 1000 ppm of nitrogen oxides on lubricants containing phenolic and aminic antioxidants, individually or in combination, in branched alkane base fluid has been investigated in laboratory reactors at 180 °C, representative of piston conditions. The reaction of the phenolic antioxidant, octadecyl 3-(3,5-di-tert-butyl-4-hydroxy-phenyl) propanoate with 1000 ppm NO2 in N2 resulted in the formation of a quinone methide product (octadecyl3-(3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dien-1-ylidene)propanoate), in apparent contradiction to previous studies, which observed at room temperature the formation of nitro-phenols. Therefore a novel reaction mechanism is proposed to account for this change in mechanism with the temperature, with the addition of NO2 to phenoxyl radicals being dominant at low temperatures, but reversible at a higher temperature,at which the dominant reaction becomes an abstraction of a hydrogen atom from the α carbon on the alkyl group at the para- position of the phenoxyl radical. The thermochemistry of key intermediates for this mechanism was also investigated using ab-initio calculation, allowing the prediction of the ceiling temperature for the addition of NO2 (1000 ppm) to phenoxyl radicals, and hence of the formation of nitrated phenolics, to be approximately 51 ± 57 °C. The reaction of the aminic antioxidant, 4.4’-dioctyl diphenylamine, with 1000 ppm NO2 was also investigated, with chemical analysis of the reaction products showing the formation of two monomers and five dimers of the starting antioxidant, and possible reaction mechanisms are based on these results. The reaction of NO2 with phenolic and aminic antioxidants when used in conjunction resulted in the identification of two new intermediates formed partially from fragments of aminic and phenolic antioxidants; novel chemical mechanisms for their formation are also suggested.