The effect of molecular composition and structure on the development of porosity in pitch-based activated carbon fibers

Seven oligomeric fractions of well-defined composition and molecular weight distribution were generated via supercritical extraction from an isotropic petroleum pitch (M-50) and used as precursors for the production of activated carbon fibers. Both isotropic and mesophase-containing fractions were produced, so that the effects of molecular order and molecular weight could be separated. Carbonization weight loss was found to gradually decrease with increasing molecular weight (and oligomeric number), with mesophase content not being a significant factor. Similar behavior was observed for activation weight loss when the precursors were isotropic; however, even modest increases in molecular order significantly retarded the activation process and resulted in dramatic drops in specific pore volume. A 100% dimer precursor fraction with an average molecular weight of 480 Da produced activated carbon fibers with the highest (specific) pore volume, and the highest pore volume in the range desired for hydrogen adsorption (6–7 Å). Even for isotropic precursors, decreases in pore volume with increasing molecular weight were observed. The incremental pore size distribution generated from the nitrogen adsorption data consisted of discrete peaks, which is consistent with the formation of pores by the removal of short micrographene layers, with each layer being formed from an individual oligomeric molecule.