Composition and Structure of Hexane-Insoluble Asphaltenes from Coal Pitch

Abstract

The functional-group composition and structure of hexane-insoluble asphaltenes from coal pitch are studied. The methods employed are chemical (elemental) analysis, nuclear magnetic resonance (NMR), IR spectroscopy, scanning electron microscopy, X-ray phase analysis, and differential thermal analysis in combination with chromatography and mass spectrometry. Analysis of the results shows that the hydrocarbon skeleton of the mean hypothetical macromolecule of the asphaltenes may be regarded as a set of condensed aromatic cores of pericondensed type, with seven rings. Alkyl substituents are present; the mean length of the connecting bridges is two carbon atoms. On average, there is one sulfur atom for each 500 carbon atoms; one nitrogen atom for each 50 carbon atoms, and one oxygen atom for each 167 carbon atoms. The ratio of structural units in asphaltene powder is as follows: for one S-bearing structure, there will be 4–5 N-bearing structures and 1.3–1.7 O-bearing structures. Sulfur atoms are present in thermally stable thiophene-like structural fragments. They enter the coke residue without decomposition. Oxygen is present in thermally unstable carboxyl and phenol groups, which break down to form CO2. It is also present in aromatic esters, which pass to the coke residue. Crystallographic analysis of the ordered structures formed by asphaltene macromolecules confirms that such macromolecules tend to undergo self-organization. The mean diameter La of such ordered structures (packets) is around 0.66 nm, while their thickness is Lc ≈ 1.30 nm. The distance between individual layers d002 ≈ 0.34 nm; the mean number of layers in the packets is n ≈ 5–7. Asphaltene powder has no more than 30% crystal structure. The data obtained by different methods are in good agreement and more completely characterize the structure of the asphaltene macromolecules in coal pitch.