Particle Size-Configuration Determination of Coal-Derived Asphaltenes and Preasphaltenes in Solution By Small Angle X-Ray Scattering.

Abstract

Coal-derived asphaltenes and preasphaltenes from a coal liquefaction residue from the H-Coal (Illinois No. 6 Coal) Process were separated by solvent extraction. The asphaltenes were further fractionated by gel permeation chromatography and chemical means into fractions of different molecular size and chemical nature. The asphaltenes and preasphaltenes were characterized by elemental analysis, vapor pressure osmometry, proton NMR and neutron activation analysis. The size and shape of the asphaltene and preasphaltene miscelles in solution were determined from small angle X-ray scattering measurements and the effects of concentration, asphaltene characteristics and molecular weight, solvent properties and agitation established. The size was determined from the scattered intensity as a function of angles. The X-ray analysis of model asphaltene compounds (o-phenylphenol, and quinoline) in solution showed that only the associated micelles scatter X-rays. The associated micelles were found to be around 29-38 (ANGSTROM). They were best fit statistically as spherical micelles. The higher the concentration of the asphaltene molecules that would associate, the greater the intensity and the greater the numbers of associated micelles. Hydrogen bonding solvents like m-cresol were found to disperse the asphaltenes and preasphaltenes while decalin promoted association, yielding more micelles and micelles of bigger size. The higher molecular weight asphaltenes with their higher heteroatom content and greater aromaticity yielded larger micelles. Asphaltenes exist as a floc under certain conditions as was evidenced by the ultrasonic agitation data. The colloidal behavior of the coal-derived asphaltenes and preasphaltenes was discussed. Heteroatom content, metal content and aromaticity are major contributing factors for association of asphaltenes and preasphaltenes in solution. Hydrogen bonding and (pi)-(pi) bonding are important in the initial association stage of asphaltenes and preasphaltenes while London-Van der Waal's attraction is more prominent in the final association stage.