Molecular structures for the oligomeric constituents of petroleum pitch

Individual dimer, trimer, and tetramer constituents that comprise the higher molecular weight (mol wt), mesophase-forming fraction of M-50 petroleum pitch were characterized in terms of polycyclic aromatic hydrocarbon (PAH) backbone structure, the extent of alkylation, and the nature of the bonds connecting the monomeric “building blocks” comprising the oligomeric species. Isolation of individual oligomeric constituents for subsequent analysis and identification was made possible by applying our two-step, sequential fractionation technique of dense-gas/supercritical extraction (DGE) followed by prep-scale gel permeation chromatography (GPC). MALDI and MALDI-post source decay (PSD) mass spectrometry of the narrow mol wt fractions obtained indicate that the major oligomeric constituents are grouped in terms of well-defined molecular weight distributions, with each composed of base monomer PAH backbone structure(s) (e.g., pyrene, triphenylene, benzopyrenes, chrysene, and benz[a]anthracene) possessing various degrees (typically 1–4/monomer unit) of methylation. UV–Vis analysis indicates that the oligomerization process occurs via a condensation reaction, with the loss of four hydrogens as a nonalternant, 5-membered PAH ring forms to join the reacting monomer units. FT-IR spectroscopy indicates that this reaction results in relatively uncondensed structures over the entire range of oligomers investigated. Analogous results to those given above were obtained for the thermal polymerization of an anthracene pitch.

Graphical abstractThermal polymerization of petroleum pitch to form mesophase occurs by the joining of monomer “building blocks’’ to form dimers, trimers (shown at right), and tetramers. This reaction occurs not by large-scale condensation, but by the net loss of only four hydrogen atoms and the formation of a single, five-membered connecting ring.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Petroleum pitch consists of alkylated PAH monomers, dimers, trimers, and tetramers. ► Oligomers consist of monomer units joined via a single, 5-membered ring. ► Little net change to the initial monomer “building block’’ unit is observed. ► Oligomers (i.e., mesophase-forming molecules) have relatively open structures.