Tailoring Mesophase Pitch from Direct Coal Liquefaction Heavy Oil by Supercritical Extraction Fractionation

Direct coal liquefaction (DCL) heavy oil, a carbon-rich material comprising over 70 wt % aromatic hydrocarbons and abundant in highly condensed polycyclic aromatic hydrocarbons (PAHs), holds significant promise as a precursor for mesophase pitch (MP), a crucial intermediate in needle coke production. However, its inherent compositional complexity and broad distribution of aromatic ring condensation degrees render it unsuitable for the direct synthesis of high-quality MP. To mitigate this heterogeneity, DCL heavy oil was subjected to stepwise supercritical fluid extraction (SFE) fractionation, and the molecular compositions of the resulting fractions were characterized. Carbonization experiments revealed three principal findings: (1) SFE facilitates targeted pretreatment of DCL heavy oil by enabling modulation of the solvent’s solvating power and effective polarity through pressure control; and (2) the MP derived from the SFE fractions exhibited markedly enhanced quality compared to that from the parent DCL heavy oil; and (3) the fraction enriched in species with double bond equivalent (DBE) values of 12–17 (corresponding to 4–6 fused aromatic rings) consistently yielded high mesophase content and excellent optical texture distribution across all tested conditions, thereby establishing it as the optimal feedstock for high-quality MP production.