Chemical Structure of Asphaltene Derived from Coal Hydrogenation Reaction
Susumu YOKOYAMA, Norihiro TAKETOMI, Masaaki SATOU, Yuzo SANADA
The chemical structure of asphaltene was investigated as a model for coal with the aim of acquiring clues to the polymerization of coal macromolecules.
Akabira coal hydrogenation asphaltene was separated according to the molecular weight of its components by means of GPC into 8 fractions (As. GPC-Fr. 1-8). The average molecular weight of fractions measured by VPO decreased smoothly from 1360 for As. GPC-Fr.1 to 200 for As. GPC-Fr. 8 with increasing fraction numbers. Regardless of the large variation in molecular weight among the GPC fractions, H/C atomic ratios and characteristic structural parameters such as fa and Hau/Ca showed much lower differences. From these results, the chemical structure of asphaltene may be considered to consist of approximately similar unit structures of clusters of 2 to 3 aromatic rings with alkyl and naphthenic ring structures of similar size. From NCa-Ncba correlation diagrams which were proposed previously, the average aromatic ring structure of units and the numbers of units making up asphaltene oligomer structure could be estimated.
Secondary hydrogenation reactions were performed on asphaltene GPC fractions and the chemical structures of product oils (O. (GPC-Fr. 1-7)) were elucidated according to the same procedure adopted to original asphaltene GPC fractions. The molecular weight of product oils diminished greatly to nearly constant values between 320 and 370 regardless of the molecular weight of the feedstock. Structural analyses also suggested that slight hydrogenation of aromatic rings occurred, causing minor changes, including a slight increase of H/C and Hau/Ca and a slight decrease of fa.
Comparison of results of structural analyses between As. GPC-Fr. 1-8 and O. (GPC-Fr. 1-7) showed that asphaltene oligomer structure degraded to 1 or 2 unit structure after rupture of linkage between aromatic units. Infrared analyses of oxygen groups suggested that the ether group is an important bonding structure between structural units and essential in the degradation of asphaltene polymers to oils.