Ja. Manion et al., DECARBOXYLATION AND COUPLING REACTIONS OF AROMATIC-ACIDS UNDER COAL-LIQUEFACTION CONDITIONS, Energy & fuels, 10(3), 1996, pp. 776-788
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.