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The combination of multistage, dense-gas extraction (DGE) and matrix-assisted, laser desorption/ionization (MALDI) mass spectrometry (MS) was investigated for the fractionation and characterization of an oligomeric model pitch synthesized via the thermal polymerization of anthracene. Our approach was shown to be effective both for elucidating the molecular properties of the anthracene pitch and for generating narrow molecular weight (MW), oligomeric cuts that can serve as MW standards. DGE was used to fractionate 10-to 15-g charges of anthracene pitch into relatively pure, gram-and decigram-sized cuts containing 99+% monomers, 90+% dimers, and 80+% trimers, respectively. The multistage, semi-batch DGE process was carried out in two steps with toluene being used as the dense-gas solvent at both subcritical and supercritical conditions. In the first step, lower-MW species were stripped off as the overhead; for the second step, the remaining residue was extracted, yielding the desired oligomeric cuts as overhead. Computer process simulation was used to guide the experimental work and demonstrated the effect of column temperature gradient on the liquid reflux ratio and, as a result, product purity. MALDI-MS played a critical role in guiding the selection of DGE operating conditions, yielding absolute MW information on the fractions that were produced.

Dense-gas extraction (DGE) was used to fractionate a petroleum pitch feedstock of broad molecular weight distribution (MWD) into selected oligomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (T c = 318.6 • C, P c = 41.1 bar) and a pitch charge of 10-15 g. A packed column with reflux produced the equivalent of several equilibrium stages. The apparatus was operated at near and supercritical temperatures, with the stillpot at 320 • C and the column temperature increasing linearly up to the reflux condenser temperature of 360 • C. Operating pressures were determined by the conditions required to extract a given oligomer and varied from 44 to 84 bar. Using a monomer-rich petroleum pitch feed (MW n = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MW n = 521, PDI = 1.07) was obtained, with the remaining residue (MW n = 883, PDI = 1.24) consisting of 95% trimer and higher oligomers. MALDI-TOF mass spectrometry was used to determine the MWD of the feed pitch and pitch fractions, with a unique matrix being used to enhance the response of higher MW species. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels.

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