Study on effect of cross-linked structures induced by oxidative treatment of aromatic hydrocarbon oil on subsequent carbonized behaviors

The variations in softening points, sub-fraction contents and carbon residue of air-blown pitches prepared from air blowing treatment of refined hydrocracking tail oil at the different conditions were monitored to select the suitable oxidative condition to prepare the feedstock of mesophase pitch. Furthermore, molecular structures of as-prepared pitches with different oxidative degree were investigated and the influence of cross-linked structural types on the carbonized performance of air-blown pitches was also discussed in terms of carbonized reactivity and planarity of fused-ring aromatic compounds formed at the initial stage of carbonization. During the early stage of oxidative treatment, the dealkylation, dehydro-aromatization of naphthenic ring and the following formation of methylene-bridged polycyclic aromatic compounds are mainly responsible for promoting the softening point of feedstock to 55 degrees C of PP23 pitch and 91 degrees C of PP38 pitch; those structural changes contribute to smoothly generating fused-ring aromatic compounds with good planarity configuration, which significantly improves the mesophase development during subsequent carbonization process: i) mesophase content of the carbonized parent pitch (MP-H) is 46% while for the carbonized oxidized pitches MP-PP23 and MP-PP38 it is 84% and 80% respectively; ii) lamellar orientation degree Og increases from 0.957 (MP-H) to 0.970 (MP-PP23 or MP-PP38) and iii) stacking height Lc increase from 2.07 nm (MP-H) to 2.94 nm for MP-PP23 and 2.98 nm for MP-PP38. With the oxidative degree deepening, the competitive cross-linked reaction featured with the formation of the polycyclic aromatics bridged by oxygen-containing functional groups such as CdbndO and OsbndCsbndO becomes dominant, whose cross-linked structure not only accelerates the carbonized reactivity but also kinetically contributes to conversing into five-member-ring bridged fused-ring aromatic compounds with poor molecular planarity, thereby leading to reducing mesophase content of MP-PP48 (prepared from PP48 pitch) and MP-PP52 (derived from PP52 pitch) to 75% and 54%, accompanying by increasing interlayer space d(002) to 0.344 nm and 0.350 nm as well as decreasing the stacking height Lc to 2.56 nm and 1.53 nm, respectively.