DECARBOXYLATION AND COUPLING REACTIONS OF AROMATIC-ACIDS UNDER COAL-LIQUEFACTION CONDITIONS

Decarboxylation of a series of monomeric benzoic acid systems under li quefaction-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 degrees C for unactivated acids to >98% fo r 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 hydro xy-activated benzoic acids results in products that are susceptible to subsequent electrophilic coupling reactions; (4) under strongly oxidi zing conditions, namely the absence of scavengers and the presence of one-electron oxidants, coupling of even unactivated acids can be as mu ch as 50% of the decarboxylations; (5) amine bases tend to promote dec arboxylation but either inhibit or do not affect coupling; (6) H-donor s inhibit coupling but promote decarboxylation in the presence of the electron transfer agent Fe3O4; (7) small amounts of water do not affec t the coupling/decarboxylation ratio; (8) rates of decarboxylation and coupling of calcium salts of aromatic acids are not significantly hig her 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 o r ion fragments produced as decarboxylation intermediates of unactivat ed aromatic acids. However, coupling of phenolic products from activat ed benzoic acids may indeed be an important cross-linking route. An ex ample 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 alipha tic acids is substantially different, as is addressed in a separate pu blication.