Controllable Synthesis of Highly Graphitizable Pitches from 1-Methylnaphthalene via Closed-System Dehydrobromination

Novel graphitizable pitches with controllable softening points and methylene-bridged structures were successfully prepared through photobromination of 1-methylnaphthalene (1-MNa) followed by closed-system dehydrobromination (CSD). The structures of bromination products and dehydrobromination pitches were determined using gas chromatography–mass spectroscopy, NMR and laser desorption/ionization-time of flight/mass spectroscopy. It was found that the amount of bromine introduced greatly affected the composition of bromination products. 1-MNa brominated at a bromine/1-MNa molar ratio of 0.75 (BMNa-0.75) demonstrated the highest methyl bromination selectivity (Sₘb), which was selected as the dehydrobromination precursor. After a CSD/polymerization reaction under 200–250 °C, dehydrobromination pitches with softening points of 148–226 °C were acquired. Compared with open-system dehydrobromination (OSD), CSD endowed bromine radical longer life and boosted intramolecular and intermolecular linking, thereby substantially increasing softening points and coking values. The polymerization degree of CSD-derived pitches is highly regulable depending on dehydrobromination temperature and the amount of tetrahydronaphthalene (tetralin) introduced. Moreover, methyl migration products (naphthalene, 2-methylnaphthalene, dimethyl-naphthalene and trimethyl-naphthalene) together with hydrogenation products (mainly tetralin) were detected by analyzing the hexane-soluble components (HS). The structural features of CSD-derived pitches contributed to some unique properties such as high polymerization degree accompanied by low aromaticity, continuous molecular weight distribution and complex connection types among subunits including α–α′, α–β′, and β–β′. The semicoke with 94% anisotropy of coarse flow texture was synthesized at 420 °C under atmospheric pressure. Well-developed graphitic carbons with graphitization degree of 81.40% and ID/IG of 0.12 was obtained from graphitization of dehydrobromination pitches under 2600 °C.