Exploring the Low-Temperature Oxidation Characteristics of Butanol Isomers in a Jet-Stirred Reactor

Butanol isomers are an important bio-alternative fuel and an important component of sustainable e-fuels. The high-temperature combustion chemistry of butanol isomers has been extensively studied, but their low-temperature oxidation chemistry remains underexplored. Detailed measurements of the species distribution are necessary for further understanding their low-temperature oxidation chemistry. This work achieved the low-temperature oxidation (365-800 K) of sec-butanol, iso-butanol, and tert-butanol in an atmospheric jet-stirred reactor with O-3 addition, and determined the detailed species distribution during their low-temperature oxidation process via synchrotron vacuum ultraviolet photoionization mass spectrometry analysis. A detailed butanol isomer model was developed to improve model predictions based on the experimental measurements. Combining the species distribution in the low-temperature oxidation of n-butanol (Liu et al. 2023) with the experimental measurements of this work, a detailed study on the low-temperature oxidation characteristics of all four butanol isomers was carried out. At lower temperatures of < 570 K, the reactivity of the four butanol isomers is similar due to their strong dependence on the thermal decomposition of ozone and the reaction of O-3 + H(O)over dot(2) = (O)over dotH + 2O(2). In the temperature range of 570-800 K, the sub-chemistry of the four butanol isomers contributes greatly to their low-temperature oxidation reactivity, and their reactivity order is n-butanol > sec-butanol approximate to iso-butanol > tert-butanol. Especially, butanol isomers can undergo alkane-like low-temperature oxidation reaction sequences, but also exhibit some special reaction pathways due to the presence of -OH group, e.g., the reaction pathways of alpha-(R)over dot + O-2 = aldehyde/ketone + H(O)over dot(2) and alpha-(Q)over dotOOH + O-2 = KHP + H(O)over dot(2) (the alpha-carbon atom is the carbon atom adjacent to the -OH group). Furthermore, at lower temperatures, the bimolecular reaction of R(O)over dot(2) radicals (R(O)over dot(2) + R(O)over dot(2) = R(O)over dot + R(O)over dot + O-2) are negligible in the n-butanol system, but become important in the other three butanol isomer systems and dominate the low-temperature oxidation of tert-butanol.