Pyrene oligomers: Their isolation, molecular characterization, and phase behavior

Carbonaceous pitches can serve as attractive precursors for advanced carbon materials such as high thermal conductivity fibers or carbon-carbon composites. Yet the effects of composition and molecular structure on the bulk properties (e.g., mesophase content and melting point) of these precursors remain only partly known, mainly because of the difficulties associated with the isolation, identification, and quantification of the large (i.e., molecular weight (mol wt) > 350-400 Da) polycyclic aromatic hydrocarbon (PAH) oligomers comprising these pitches. In this study, oligomeric fractions were isolated via supercritical extraction (SCE) from two representative pitches: a petroleum pitch (M-50) produced by the thermal polymerization of aromatic decant oil (a byproduct of the fluid catalytic cracking process) and a pyrene pitch produced catalytically with AlCl3. Molecular-structure characterization and mesophase analysis were performed on selected oligomeric fractions and oligomers. High-purity (>97%) dimer and trimer fractions were isolated from M-50 pitch via SCE using pure toluene as the extractive solvent. 99%-pure dimer and trimer oligomers were isolated from catalytically polymerized pyrene pitch; however, their recovery was only possible with the addition of N-methylpyrrolidone (NMP) as a cosolvent with toluene. This solvent mixture significantly increased product yields (3x vs. neat toluene) and inhibited undesirable side reactions that occurred between pure toluene and pyrene oligomers. The key dimeric isomers present in pyrene pitches produced both catalytically and thermally were isolated from the dimer fraction via reverse-phase HPLC, using dichloromethane (DCM) as the mobile phase. Molecular-structure information for individual species was obtained via MALDI-MS and by UV-vis and fluorescence spectroscopy. With this approach, the 5 possible pyrene dimer isomers (2 alternant and 3 non-alternant) in the catalytically produced pitch were all isolated and identified. Furthermore, four of those isomers (2 alternant and 2 non-alternant) were also identified in the pitch produced thermally. Methylated species were detected only in the pitch made catalytically, indicating that the methylation reactions occur exclusively because of catalytic activity. Both pyrene trimer and an M-50 trimer fraction were analyzed for mesophase via cross-polarized optical microscopy. A fully developed (100%) liquid crystalline phase was observed for the pyrene trimer (mol wt = 598 Da) produced catalytically. To the author's knowledge, this is the lowest molecular weight PAH for which mesophase formation has ever been observed. Furthermore, the softening point of the pyrene trimer mesophase was a relatively low 290 ºC. On the other hand, conversion to mesophase (~40%) was observed for the M-50 trimer fraction, which had a mol wt range of 650-900 Da.