Hydrogenation Behaviors and Structural Variations of Subfraction of Coal Tar Pitch and Preparation of Mesophases with Well-Developed Optical Textures

The heavy fraction (P-A) and the medium fraction (P-B) of coal tar pitch (CTP) were independently hydrogenated by tetrahydronaphthalene (THN) and then blended with the light fraction (P-C) of CTP to obtain modified pitches, from which a fully developed mesophase was prepared. The polycondensation behavior of the hydrogenated fractions and their influence on modified pitches forming mesophase were investigated. The results showed that after P-A fraction was hydrogenated by 10-40 wt% of THN, the H/C atomic ratio increased from 0.57 to 0.59-0.65, precisely as the content of naphthenic structures increased with the condensation index increasing from 0.488 to 0.568. For the hydrogenated P-B, when the ratio of THN increased from 5 wt% to 15 wt %, the alkyl side chains tended to shorten and then grow with their average chain length parameter L decreasing from 3.95 of P-B to 3.81 and then increasing to 4.46. The content of mesophase prepared from modified pitch ranged from 64.33% to 85.65%, in consequence of obvious synergistic effect of the hydrogenated P-A and P-B. Benefiting from the similar condensation degree and average chain length between hydrogenated P-A and P-B, the mesophase content of modified pitches reached 85.65%, including 75.96% of the fiber and flaky textures. The polycondensation reaction between fractions with similar molecular structure proceeds uniformly and smoothly in a well-flowing pitch matrix, resulting in a high content of wide-area mesophase ultimately, which can lay the foundation for the subsequent preparation of high-performance carbon materials.