(84f) Fractionation and Characterization of Pyrene Pitch Oligomers

Carbonaceous pitches can be used as precursors for a variety of high-performance carbon materials, including cathodes for lithium ion batteries, high thermal conductivity carbon fibers, and carbon-based composites. These pitches can be produced from various resources, such as coal tar, aromatic decant oil, and pure polycyclic aromatic hydrocarbons (PAHs) using either thermal or catalytic polymerization processes. Thies and co-workers have developed a method for fractionating thermally polymerized, petroleum-based pitches into narrow molecular weight (mol wt) cuts via dense-gas (also called supercritical) extraction (DGE or SCE). DGE can in principle be applied to all types of pitches, regardless of their method of production. Fractionating pitches in order to isolate the individual oligomers for study is a useful technique for helping us develop a better understanding of the molecular structure–bulk property relationships of pitches and their consituents.

In this study, pyrene pitch was fractionated into its constituent oligomers via DGE using N-Methyl-2-pyrrolidone (NMP)/toluene mixtures as the extractive solvent. The fractionation of synthetic, oligomeric pitches derived from PAHs such as pyrene is strongly limited by their lower solubilities in traditional aromatic solvents, such as toluene.  The addition of NMP as a co-solvent to toluene was found to significantly increase the solubility of the pyrene pitch and allowed for the extraction of oligomers up to trimer.

The experimental setup consisted of a 17-mm i.d. x 2-m long packed column operated in a semi-continuous mode, in which solvent flows continuously through a pitch charge. A positive temperature gradient along the column from bottom to top (330, 350, and 380 °C) was used to enhance selectivity. NMP/Toluene mixtures of up to 30 mol % NMP and pressures of up to 80 bar were used to obtain fractions of different mol wt distributions and purities. The use of pure toluene under supercritical conditions (Tc = 321 °C, Pc = 42.2 bar) resulted in poor yields and the extraction of only monomer and low mol wt oligomers (i.e., dimer), whereas supercritical NMP/toluene mixtures resulted in higher yields and the extraction of oligomers up to trimer. The critical temperature and pressure of pure NMP have been estimated as Tc = 447 °C and Pc = 45.8 bar.

To our knowledge, this study represents the first time that NMP/toluene mixtures under supercritical conditions have been used for the fractionation of high mol wt PAHs such as pyrene pitch, and that pyrene oligomers up to and including trimer have been isolated. These results represent a significant step towards producing pyrene oligomeric standards (i.e., fractions of high purity) that can be used for establishing composition–property relationship in carbon materials.