Coupling pathways for dihydroxy aromatics during coal pyrolysis and liquefaction

Studies with model-compounds and hybrid studies in which model compounds are added to coal experiments were conducted to elucidate the major pathways by which phenols can lead to cross-links during coal conversion. This effort was greatly facilitated by the application of field ionization mass spectrometry (FIMS). For 1,3-dihydroxynaphthalene, FIMS analysis showing the entire sequence of oligomeric coupling products up through pentamers at 688 Da demonstrates unequivocally that the stoichiometry and sequence of reactions at 400 °C involve an initial condensation to eliminate water, followed by an oxidative coupling eliminating the elements of H2. Reaction conditions were varied to determine the effect of neutral and basic hydroaromatic solvents, non-donor aromatic solvents, reactor surfaces, metal-oxide additives, and gas atmosphere. Resorcinol (1,3-dihydroxybenzene) couples in a similar manner, but analyzable products are difficult to intercept; evidently the initial coupling products are far more reactive toward coupling than is the starting material. Catechol (1,2-dihydroxybenzene) undergoes self-coupling at a slightly slower rate than resorcinol, while 1,4-dihydroxynaphthalene undergoes dehydroxylation in donor solvents at a rate fast enough to largely block rapid self-coupling of the dihydroxy aromatic. Hybrid experiments with a subbituminous coal (Wyodak) and these Ar(OH)2 show that the coal possesses structures that couple far more rapidly with Ar(OH)2 than the latter do with themselves. Simple phenols inhibit the coupling, but the amine hydroaromatic, tetrahydroquinoline, has no special inhibiting effect, except that it itself tends to couple with Ar(OH)2.