Structural Identification of the Monomeric Constituents of Petroleum Pitch

The monomer fraction of a representative petroleum pitch, M-50, was characterized in terms of both the predominant polycyclic aromatic hydrocarbon (PAH) backbones and their extent of alkylation. Such an identification of individual molecular species and their relative abundance was greatly facilitated by the use of a series of separation processes prior to analysis. In particular, low (up to 270), medium (270-335), and high (335-388) molecular-weight cuts of the monomeric fraction of the M-50 pitch were isolated by dense-gas extraction (DGE). The resultant three cuts were then further fractionated by preparative-scale gel permeation chromatography (prep-scale GPC) to obtain narrow molecular-weight fractions suitable for individual species identification by both matrix-assisted laser desorption/ionization (M ALDI) time-of-flight mass spectrometry and reversed-phase high-performance liquid chromatography with photodiode array detection (HPLC/PDA). Results indicate that the monomer fraction consists of five dominant series, with each series consisting of 1-3 PAH backbones typically substituted with 0-4 methyl groups. The dominant PAH backbones include pyrene (molecular weight = 202); chrysene, benz[a]anthracene, and triphenylene (molecular weight = 228); benzo[a]pyrene and benzo[e]pyrene (molecular weight = 252); and benzo[ghi]perylene (molecular weight = 276). The highest molecular-weight series beginning at 326 could not be unambiguously identified but may in fact be a small dimer formed by the condensation reaction of pyrene and naphthalene in the starting decant oil, because the UV-vis spectra indicate the presence of a five-membered ring, something not observed in any of the other identified series.