Structural characterization of gilsonite bitumen by advanced nuclear magnetic resonance spectroscopy and ultrahigh resolution mass spectrometry revealing pyrrolic and aromatic rings substituted with aliphatic chains

Abstract

Gilsonite, a naturally occurring asphaltite bitumen, consists of a complex mixture of organic compounds. In the present study, advanced one and two dimensional solid state and solution 1H, 13C and 15N nuclear magnetic resonance (NMR) and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) were employed to investigate its composition and structure. 13C NMR yielded a carbon aromaticity of 27%. Aromatic moieties in gilsonite were primarily single rings or small clusters of fused rings. Half of the aromatic carbons of gilsonite can be accounted for by pyrroles. 15N and 13C cross polarization-magic angle spinning (CP-MAS) NMR showed that most nitrogen in gilsonite was pyrrolic. The aromatic rings were heavily substituted with alkyl chains, as evidenced by 1H13C correlation spectra. Advanced solid state NMR spectral editing techniques clearly identified specific functional groups such as CCH3, CCH2, and CCH2 (exomethylene). 1H13C wideline separation (WISE) NMR helped identify mobile and non-protonated alkyl carbons. FT-ICR-MS indicated that ∼64% of calculated formulae generated by ESI were aliphatic, while only about 0.8–2.5% of formulae contained possible aromatic rings. All of the assigned formulae contained at least one heteroatom (N, O or S), indicating that ionization by ESI was selective for the polar fraction of gilsonite and potentially less reflective of the overall chemical character of gilsonite than NMR spectroscopy. By combining the information obtained from advanced NMR and ultrahigh resolution MS we propose a structural model for gilsonite as a mixture of many pyrrolic and a few fused aromatic rings highly substituted with and connected by mobile aliphatic chains.