Fractionation of an oligomeric pyrene pitch via supercritical extraction

Semi-continuous, packed-column supercritical extraction was investigated for the fractionation of a polycyclic aromatic hydrocarbon (PAH) pitch into its constituent oligomers. The starting pitch had been produced by the catalytic polymerization of pure pyrene using AlCl3. Although neat supercritical toluene (Tc = 318.6 °C; Pc = 41.1 bar) was shown to be an effective solvent for the recovery of both monomer and dimer cuts in high purities, higher-oligomer solubilities in toluene are notoriously low. However, the addition of N-methyl-2-pyrrolidone (NMP) as a co-solvent enhanced oligomer solubilities in the extractive, supercritical-solvent phase by a factor of 3, making possible the recovery of trimer and higher oligomers. An unexpected benefit of the use of NMP is that it was found to suppress the condensation reaction between toluene and the pyrene oligomers – apparently by forming a Lewis acid–base complex with the AlCl3 catalyst – that can occur in the extraction column at temperatures above 350 °C.

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► Supercritical extraction was investigated for fractionating a pyrene pitch into its constituent oligomers.
► Pure monomer and dimer cuts can be obtained with supercritical toluene.
► When NMP is added as a co-solvent, pure trimer and tetramer cuts can also be isolated.
► NMP suppresses the undesirable toluene–oligomer condensation reaction via AlCl3–NMP complexing.
► Mass balance around the extraction column gives a monomer composition of 30 ± 1 wt% of the starting pitch.