Gas-phase reaction of OH radicals with benzene: products and mechanism

Abstract

The gas-phase reaction of OH radicals with benzene was studied in O₂/He mixtures under flow conditions in the temperature range 276–353 K and at pressures of 100 and 500 mbar using on-line FT-IR spectroscopy and GC-MS measurements. The reaction conditions were chosen so that the initially formed OH/benzene adduct predominantly reacted either with O₂ or O₃. Under conditions of a predominant reaction of the OH/benzene adduct with O₂ the product formation was studied for variable NO concentrations. Identified products were the isomers of hexa-2,4-dienedial, phenol, nitrobenzene, p-benzoquinone and glyoxal. Furan was found in small amounts. For increasing NO concentrations there was a decrease of the phenol yield and the yields of trans,trans-hexa-2,4-dienedial and nitrobenzene increased, resulting in maximum values of 0.36 ± 0.02 and 0.11 ± 0.02, respectively (100 mbar, 295 K). The p-benzoquinone yield of 0.08 ± 0.02 was found to be independent of the NO concentration. The temperature dependence of the phenol yield was measured in the range of 276–353 K for initial ratios of [NO]/[O₂] = 1–20 × 10⁻⁶ at 500 mbar. For a fixed [NO]/[O₂] ratio, a distinct increase of the phenol yield with increasing temperature was observed; initial [NO]/[O₂] = 1–1.2 × 10⁻⁶, phenol yield: 0.18 ± 0.04 (276 K) and 0.68 ± 0.05 (353 K). Generally, the total yield of carbonylic substances was found to be anti-correlated to the phenol yield. When the OH/benzene adduct reacted with O₃, trans,trans-hexa-2,4-dienedial, phenol and formic acid were identified as main products with formation yields of 0.28 ± 0.02, 0.20 ± 0.05 and 0.12 ± 0.02, respectively (100 mbar, 295 K). Further products were p-benzoquinone, CO and unidentified carbonylic substances. For the different experimental conditions, reaction mechanisms are proposed explaining the formation of the observed products. A simple model describing the temperature and NO_x-dependence of the phenol yield is presented.