Molecular-level investigation of coal-tar pitch treated by air blowing: Revealing the restructure of aromatic compounds via radical reactions

초록

Air blowing is conventionally used to improve the properties of pitch via oxidation. Previous studies have shown that pitch modification by air blowing affords considerably higher carbon yield, without restricting the fluidity necessary for impregnation of pitch. This has been attributed to the dehydrogenative polymerization and crosslinking of aromatic molecules during the air-blowing process. However, in this study, during air-blowing experiments, samples were obtained in similar yields but with higher softening points despite the absence of larger aromatic compounds. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used for the advanced analysis of pitch, enabling us to achieve a more complete understanding of the mechanism of coaltar pitch reforming via air blowing. The results show that air blowing does not simply induce polymerization of oxygen radicals but converges aromatic compounds to more stable congeners via radical reactions. The newly identified facts suggested herein can be summarized as follows. 1) Radical stability determines the reaction pathway. 2) Carbon radicals are derived from the removal of carbon branches. 3) Nitrogen radicals are generated by the removal of aliphatic carbons. 4) Oxygen radicals originate from the alcohol group formed by the oxygen atom supplied during oxidation. 5) Thus-generated radicals are either removed or bridged to afford highmolecular-weight aromatic compounds.

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