Sorption behaviors of methanol vapor by coal extracts and residues

Four Argonne coal samples (Pocahontas No.3, Upper Freeport, Illinois No.6, and Beulah-Zap coal) were extracted with a carbon disulfide–N-methyl-2-pyrrolidinone (CS2–NMP) mixed solvent at room temperature. To clarify the nature of the coal–methanol interaction, the mechanism of sorption, and the micropore and bulk structure of each coal fraction, sorption of methanol vapor by the extract fractions and residues was investigated using an automatic vapor adsorption apparatus. The sorption behavior of the residues with the low extraction yields was similar to that of the raw coals, regardless of rank. In contrast, the sorption for residues with the high extraction yields greatly increased compared to the raw coals, which suggests that more microporosity was formed by the extraction. For all fractions examined, experimental values were fit by the Langmuir–Henry dual-mode sorption model, which suggests that methanol sorption by the coal fractions can be explained by the adsorption on the surface described by a Langmuir isotherm and penetration (dissolution) into the bulk described by Henryʹs law. The Henryʹs dissolution constant, kD, was found to be similar in magnitude for both the extract fractions and residue from Upper Freeport coal, in agreement with the results of methanol swelling.