Prediction of liquid crystalline content and molecular structures present in carbonaceous pitches.

摘要

Previous research at Clemson has shown that multistage, packed column, supercritical extraction (also called dense-gas extraction, or DGE) of petroleum pitches is a promising technique for the production of carbonaceous precursors that can be processed into a variety of carbon products, including activated carbons and high thermal conductivity carbon fibers. As the existence (or lack thereof) of a liquid crystalline phase, or mesophase, plays a key role in establishing the suitability of a potential precursor material for a given application, we developed the SAFT-LC (liquid crystal) equation of state by combining Maier-Saupe theory for multicomponent mixtures with the SAFT equation. SAFT-LC was used with some success to predict the effect of temperature and pressure, as well as pitch and solvent composition, on the formation of mesophase at both supercritical and ambient conditions. Unfortunately, the lack of information about the actual molecular structures present in petroleum pitch hindered the development of an appropriate set of pure-component parameters for use with SAFT-LC. Thus, the second half of this dissertation focused on structural characterization.;Previous efforts to characterize the molecular structures of the major species present in pitches have been limited by an inability to fractionate the pitch into cuts of narrow molecular weight (mol wt). However, by using DGE followed by preparatory-scale gel permeation chromatography (prep-scale GPC), we are now able to fractionate petroleum pitch into its constituent oligomers. Subsequent analytical characterization of these oligomers using high-performance liquid chromatography with photodiode array detection (HPLC/PDA), matrix-assisted, laser desorption and ionization, time-of-flight mass spectrometry (MALDI), MALDI-post source decay (PSD), and UV-Visible spectrophotometry (UV-Vis) has determined that M-50 monomer is primarily comprised of benzenoid, polycyclic aromatic hydrocarbon (PAH) "backbones" (the most prevalent of which are pyrene, chrysene, benz[a]anthracene, triphenylene, benzo[a]pyrene, benzo[e]pyrene, and benzo[ghi]perylene), substituted with from 0 to 4 alkyl (primarily methyl) groups. The most prevalent dimers are formed from the condensation reaction of two of the most prevalent monomer units, such that four hydrogens are lost and a five-membered, connecting ring is formed to create a fluoranthenoid PAH. For trimers and tetramers, MALDI, UV-Vis, and transmission FT-IR results are all consistent with the linkage of the most prevalent lower-order oligomeric units via a single, five-membered ring. Thus, the body of evidence indicates that the highly PAH-condensed structure previously proposed for such pitches does not exist.