Decarboxylation and Coupling Reactions of Aromatic Acids under Coal-Liquefaction Conditions

Abstract

Decarboxylation of a series of monomeric benzoic acid systems under liquefaction-relevant conditions was studied. The principal findings are the following: (1) decarboxylation rates of benzoic acids range from a few percent in 1 h at 400 °C for unactivated acids to >98% for species activated by OH in the ortho or para positions; (2) coupling of unactivatiated benzoic acids as a direct result of decarboxylation tends to be very minor (generally <10%); (3) decarboxylation of hydroxy-activated benzoic acids results in products that are susceptible to subsequent electrophilic coupling reactions; (4) under strongly oxidizing conditions, namely the absence of scavengers and the presence of one-electron oxidants, coupling of even unactivated acids can be as much as 50% of the decarboxylations; (5) amine bases tend to promote decarboxylation but either inhibit or do not affect coupling; (6) H-donors inhibit coupling but promote decarboxylation in the presence of the electron transfer agent Fe3O4; (7) small amounts of water do not affect the coupling/decarboxylation ratio; (8) rates of decarboxylation and coupling of calcium salts of aromatic acids are not significantly higher than those of the free acids. It is suggested that the increase in low-temperature coupling associated with the increased oxygen content of low-rank coals is not directly caused by the reaction of radical or ion fragments produced as decarboxylation intermediates of unactivated aromatic acids. However, coupling of phenolic products from activated benzoic acids may indeed be an important cross-linking route. An example of this secondary cross-linking is provided by the formation of xanthenes from the cresols generated in the decarboxylation of anisic acid. Furthermore, the decarboxylation and coupling behavior of aliphatic acids is substantially different, as is addressed in a separate publication.