Original preasphaltenes and asphaltenes in coals

Abstract

The preasphaltene (PA) and asphaltene (A) fractions from untreated coal show a direct relationship of PA/A (w/w) to the carbon content of coal. Their thermal stability was investigated by in-situ pyrolysis FTIR, and the results show that PA has higher thermal stability than A. By means of a vacuum FTIR method, five types of hydrogen bonds formed by hydroxyl groups were clearly observed in PA and A, i.e., OH- π, self-associated OH, OH–ether oxygen, cyclic OH groups, and OH–N. The self-associated OH and OH–ether oxygen are the two main hydrogen bonds. The insolubility of PA in benzene and A in hexane is determined by the hydrogen bond strengths of the acid/base fraction but is not strongly related to their structural parameters and molecular weight. The acid/base associated strength of preasphaltenes and asphaltenes is estimated to be within the following ranges, asphaltenes, <5.15 kJ/mol; preasphaltenes, 5.15–30.9 kJ/mol.