Structural Modification of Petroleum Pitch Induced by Oxidation Treatment and Its Relevance to Carbonization Behaviors

Structural modifications of petroleum pitch and its subfractions induced by mild air-blowing modification have been monitored by elemental analysis, average molecular weight, Fourier transform infrared spectrometer (FTIR), H-1 nuclear magnetic resonance (H-1 NMR), and carbon residue. Combined with the investigation into carbonization behaviors of as prepared oxidized pitch, the relationship between oxidized modification and carbonization behaviors has also been discussed. With the air blowing treatment process, average molecular weight and oxygen content grow rapidly in the petroleum ether insoluble fraction of the air-blown pitch in comparison with those of corresponding petroleum ether-soluble fractions, which gradually enlarges the distinction in carbonization reactivity and mutual solubility between these two subfractions. As a result, although the yield of carbonized residue is dramatically increased from 36.1 wt % to 64.5 wt % during the direct thermal carbonization process, the air blowing treatment produces an adverse effect on mesophase development of oxidized pitches, leading to the optical texture index (OTI) significantly decreasing from 35.9 to 0.6. While adding the hydrogen-donor aromatic oil (HAO) into the air-blown pitches is able to effectively improve the mutual compatibility among the solvent subfractions via H-transfer reactions and dilution effect, consequently contributing to mesophase development during the co-carbonization process accompanied by increasing OTI value of mesophase residues to about 65, except 33.6 of the PP2S-HAO mixture, simultaneously the co-carbonization process still maintains the relatively high carbonized residue yield between 42.1 and 61.8 wt %. Hence, a possible combined process including air blowing modification and subsequent co-carbonization process can obtain mesophase products with high residue yield and excellent anisotropic texture by controlling the degree of oxidation and selecting compatible co-carbonization additives.