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42722-AC4
The Molecular Design of Petroleum Pitches for Advanced Carbon Materials
Petroleum pitches serve as precursors for advanced carbon materials, such as high thermal conductivity carbon fibers and the matrix phase of carbon-carbon composites. These pitches are fascinating materials, being one of the few complex mixtures of species ever reported to exhibit liquid crystalline phase behavior. Because the properties of the final carbon products are highly dependent on the molecular composition of the precursor pitch, it would be desirable to both measure and control this composition. Thus, the objectives of this research project are to (1) use our dense-gas extraction (DGE) process to produce oligomeric pitch cuts of narrow and well-defined molecular weight distribution (MWD); (2) quantitatively establish the intensity response of matrix-assisted, laser desorption/ionization time-of-flight (MALDI) mass spectrometry to petroleum pitch moieties; and (3) determine representative molecular structures for pitch oligomers.
Our DGE unit consists of a multistage, countercurrent extraction column, with the pitch of interest being fed at the top of the column in the molten state and the dense-gas toluene solvent being fed in at the bottom. The column also contains a hot finger at the top to effect precipitation of a liquid phase, and thus reflux, back down the column, enhancing product purity. For the feed pitch, we are currently using a commercially available, isotropic pitch with a broad MWD, Marathon M-50. Changes in both the column pressure and temperature profile are being used to effect the fractionation of pitches into their constituent oligomers. Last year, we demonstrated how DGE can be used to control the oligomeric composition of bottom-phase pitch fractions and, as a result, produce pitches with a wide range of properties in varying yields. Nevertheless, 1-column DGE has an inherent disadvantage: any top product will always contain the lightest species in the feed pitch (which inhibit mesophase formation), and any bottom product will always contain the heaviest species (which can be infusible). Thus, we are investigating 2-column DGE for isolating middle cuts.
Our current focus is on producing a cut rich in dimer species. An example of a recent 2-column run is given in the figure below. Here we see that the first column was operated with a positive temperature gradient and a pressure of 76.8 bar. The normalized MALDI spectra for all process streams associated with column 1 are also given in the figure, including the M-50 pitch feed (black line) and the top (green line) and bottom (blue line) products. Here we see that the top product is concentrated in monomer and dimer, and the bottom product in trimer and higher oligomers. The top product from column 1 was then fed to the second column, which was operated at 350 °C and 52.7 bar. MALDI spectra of the desired bottom product (blue line) indicate that a fraction rich in dimer is isolated, with all monomer being removed as top product (not shown). Comparison with results obtained from one-column DGE (red line) clearly indicate the superiority of the two-column setup for concentrating the dimer and removing the heaviest species.
Impact: Most funding in the area of pitch-based carbon materials focuses on the applications side, i.e., on the manufacture of the carbon fibers and carbon–carbon composites themselves. PRF funding is allowing us to take a fundamental look at changes to the underlying precursors.
