The determination of the void structure of microporous coals by small-angle neutron scattering: Void geometry and structure in Illinois No. 6 bituminous coal

Abstract

The access of solvents and reactants to the microvoid volume in porous materials such as coal plays an important role in determining the overall chemistry which takes place during a variety of chemical transformations including oxidation, combustion, and pyrolysis. The structure and surface composition of these voids were studied using small-angle neutron scattering techniques to examine selectively the subset of the overall void volume distribution which comprises the microvoid volume. Powdered Illinois No. 6 coal containing approximately 20% void volume was slurried in several different aqueous and cyclohexane solutions. The solutions used had various hydrogen-to-deuterium ratios in order to contrast match most of the open pore volume thereby making the microvoid volume visible. The microvoid volume observed is characterized as elongated voids having a fairly well-defined diameter and surface composition. The scattering intensity from the microvoid volume shows a well-defined Porod region, indicating that the smallest void dimension is resolved by the instrumental configuration employed. A Guinier region exhibiting Q−1 behavior, which is characteristic of elongated structures, is also observed. The average radius of a circular cross section of these voids is found to be 25.4 Å. The microvoids are found to be completely filled by aqueous solutions so that the residual neutron scattering, which is not eliminated by the contrast-matching aqueous solution, is due to the organic matrix structure. Nonaqueous mixtures of cyclohexane cannot fill the entire microvoid volume as effectively as the aqueous mixtures. The scattering differences observed between the aqueous and nonaqueous filled coal indicates that the surface of the microvoids is predominantly aliphatic in character with the principal compositional variation being the presence or absence of acidic functionality on the surface.