Mechanism for the catalytic thermal polycondensation of naphthalene at low temperature

Naphthalene is an important component of high temperature coal tar and its content can reach more than 10%. Catalytic polycondensation of naphthalene is an effective way to prepare mesophase pitch and functional carbon materials. In this work, anhydrous AlCl₃ was used as a catalyst for the polymerization of naphthalene under atmospheric pressure below 170 ℃ and the reaction mechanism was then systematically investigated. The results indicate that at 110 ℃, the polymer product is mainly composed of tricyclic compounds and the content of heavy products is only 29.5%. At 150 ℃, four to five peri-condensed aromatic compounds turn to be the main components and the content of medium components remains about 50%. At 170 ℃, there appear a large number of six-ring aromatic cores and the conversion of naphthalene reaches 90.7%. The polymer products exhibit good fluidity and solubility in THF, which can facilitate the high-temperature thermal polycondensation and subsequent graphitization process. With an AlCl₃/naphthalene molar ratio of 1/100, the second to seventh order naphthalene oligomers are obtained by the simulation of the short chain oligomerization of naphthalene. In contrast, when the AlCl₃/naphthalene molar ratio exceeds 10/100, acetylene and methylnaphthalene are produced by the catalytic pyrolysis of naphthalene. The mechanism of “Oligomerization-Pyrolysis-Aromatization” was then proposed to explain the molecular transformation from naphthalene to pitch, which should be useful for the production of mesophase pitch precursor.