On the autoxidation of terpenes: Detection of oxygenated and aromatic products

Abstract

Limonene-O2-N2 and α-pinene-O2-N2 mixtures were oxidized in a jet-stirred reactor at atmospheric pressure, in the cool flame regime, and fuel-lean conditions. Samples of the reacting mixtures were analyzed by on-line Fourier transform infrared (FTIR) and collected, dissolved in acetonitrile for analysis by flow injection or chromatographic separation by ultra-high performance liquid chromatography and Orbitrap mass spectrometry. OH/OD exchange using D2O and reaction with 2,4-dinitrophenylhydrazine were carried out for probing the existence of hydroxyl or hydroperoxyl, and carbonyl functions in the products, respectively. A large number of oxidation products, including highly oxygenated organic products with more than ten oxygen atoms, were observed. Unexpectedly, aromatic and polyunsaturated products with a contribution of 8–10% were detected for both terpenes over the range of temperatures studied. Van Krevelen plots, computed oxidation state of carbon, aromaticity index, and aromaticity equivalent index in products were used to rationalize the results.