Acid-catalysed depolymerization of coal model compounds and subbituminous coal in a superacid-isopentane medium

Acid-catalysed reaction of selected coal model compounds in trifluoromethanesulfonic acid (TFMS) in the presence of isopentane at 30–150°C was studied, to clarify the catalytic function of TFMS in coal depolymerization reactions. Diphenylmethane (DPM) and benzyl phenyl ether (BPE) were almost completely converted, producing alkylated monocyclic compounds at 30°C. Compounds with a phenolic hydroxyl group easily reacted with benzyl cations, leading to a recombination reaction. In contrast, even at 150°C diphenyl ether (DPE) and biphenyl (BP) gave conversion of only 27 and 86% respectively, suggesting that aromatic ether and phenyl-phenyl linkages are more difficult to cleave in the superacidic medium. Although only 14wt% of Taiheiyo coal is pyridine-soluble (PS), 34wt% of this coal was solubilized by reaction with TFMS in the presence of isopentane at 30°C for 3 days. The increased solubility is mainly ascribed to cleavage of the methylene bridges in the coal. Reaction at 150°C increased the solubility in pyridine to 90wt%. The partial cleavage of phenyl-phenyl and aryl-aryl ether bonds at 150°C contributes to this increase in solubility.