From Asphaltenes and Coals to Carbons: Interpreting the Transition from X-ray Scattering to Crystalline Diffraction

One of the first instrumental techniques applied to asphaltenes and coal to obtain structural information was X-ray diffraction. The broad asymmetric peaks were interpreted on the basis of the graphite crystalline structure, and the resulting lattice spacings and crystallite dimensions have been published regularly for over 60 years. This review examines the basis for this analytical method and tests the underlying assumptions and hypotheses by comparison to X-ray patterns from model compounds and amorphous polymers. These comparisons show that the X-ray patterns of complex asphaltenes are dominated by inter- and intramolecular scattering and that the published methods to quantitatively analyze these data are invalid. High-temperature processing of asphaltenes and coals can give higher order X-ray peaks consistent with graphite, but prior to this, the structural information is highly ambiguous. The use of full width at half maximum for the broad signal from 10° to 30° is recommended for tracking thermal conversion reactions of such highly complex materials as asphaltenes and coal, to avoid unjustified assumptions of the crystalline structure.