Study on the aggregation of coal liquefied preasphaltene in organic solvents by UV–vis and fluorescence spectrophotometry

Aggregation of heavy products of coal liquefaction such as preasphaltene is one of important influencing factors on the coking properties of liquefaction technology. In this paper, the aggregation of coal-derived PA in tetrahydrofuran and N-methyl-2-pyrrolidinone has been investigated by gel chromatography, absorption and fluorescence spectroscopy. The results indicate that PA is a complex mixture with lower H/C (atomic ratio 0.61) and contains different chromophores and fluorophores. It shows strong propensity of aggregation and can form nanoaggregates even in the very dilute solution such as 6.25 mg/l. With the increasing of PA concentration, the concentration of aggregates increases but the dimension of aggregates is basically invariable. Although a flexion point of the fluorescence intensity integrated can be observed with increasing of PA concentration, it cannot present the concentration where any aggregation starts. It varies with the excitation wavelength.