Structural analysis of Athabasca maltenes fractions using distortionless enhancement by polarization transfer (DEPT) related 13C n.m.r. sequences

Abstract

A combination of inverse-gated heteronuclear decoupling and DEPT (including DEPT-quarternary) pulse sequences has been utilized for the assignment of the different types of carbons and for the determination of their relative importance in different molecular weight (MW) fractions separated from Athabasca maltenes by gel permeation chromatography (g.p.c.). The results obtained permit the g.p.c. fractions to be divided into two classes: MW < 800 (‘oils’) and MW > 800 (‘resins’). The lowest molecular weight fraction (MW = 315) had a quaternary and tertiary aromatic carbon content of more than 35%. Cyclic methylene and methine were also abundant and represented nearly 30% of the total carbon. The remaining carbons were methyls and short chain alkyls (the long chain alkyl carbon content was very low). With an increase in MW to 800 the aromatic carbon concentration decreased rapidly whereas the amount of carbon in saturated rings increased substantially. There was also a concomitant decrease in methyl carbon and an increase in long chain alkyl carbon content. In the case of Athabasca maltene g.p.c. fractions with MW greater than 800 the changes in structural parameters were slight. These results demonstrate that conventional 13C n.m.r., in combination with DEPT-related sequences, is a convenient tool for characterization of the bitumen constituents. Such knowledge is essential for the conversion of bitumen into synthetic crude oil and other products.