Addition of benzene and toluene to slowly reacting mixtures of hydrogen and oxygen at 773K

Abstract

Studies have been made of the separate addition of small amounts (0.05–0.5%) of benzene and toluene to slowly reacting mixtures of H 2 +O 2 at 773K in aged boric-acid-coated vessels in an attempt to elucidate the mechanism of the oxidation of aromatics and to obtain rate constants for some of the elementary reactions involved. With this system, the oxidation is carried out in a constant and reproducible radical environment controlled by the H 2 +O 2 reaction, so that toluene and benzene are oxidised under effectively the same conditions. With toluene, the major initial products are benzaldehyde, benzene, methane and carbon monoxide, with small amounts of formaldehyde, ethylene, vinylacetylene, together with traces of acetylene. Kinetic studies over a wide range of mixture composition show that toluene is removed mainly by H atom abstraction from the side group and by H addition at the carbon atom containing the CH 3 group, the adduct breaking down rapidly to give benzene. Rupture of the aromatic ring is relatively unimportant in the early stages of reaction, only becoming important when the product benzene is oxidised. Several rate constants have been obtained including k 21 =(2.85±0.35)×10 9 , k 22 =(5.8±0.76)×10 8 , and k 22 m =(3.0±0.5)×10 8 dm 3 mole −1 s −1 at 773K. OH+C 6 H 5 CH 3 →products H+C 6 H 5 CH 3 →products With benzene, carbon monoxide is the only major initial product, but methane is formed in surprisingly large amounts (up to 30% on a mole/per mole benzene consumed basis). Small amounts of formaldehyde and C 2 –C 4 hydrocarbons are found, and small amounts of phenol were detected but not estimated quantitatively. Kinetic studies suggest that about 50% of the benzene is removed by O atom addition and 50% by H atom abstraction. Although the products clearly arise from ring rupture, the kinetic results show that about 15–20% of the phenyl radicals formed react with H 2 to regenerate benzene in a stoichiometric H 2 +O 2 mixture at 773K. A preliminary value of k 28 =3.2×10 8 dm 3 mole −1 s −1 has been obtained.