Pyrolysis of aromatic carboxylic acids: Potential involvement of anhydrides in retrograde reactions in low-rank coal

The pyrolysis of 1-(3-carboxyphenyl)-2-(4-biphenyl)ethane (3) has been studied in the liquid phase at 400 degrees C neat and diluted in hydrogen-donor and nondonor solvents to determine the role of decarboxylation of aromatic carboxylic acids in the cross-linking processes in low-rank coal. Decarboxylation was the dominant reaction pathway in the pyrolysis of this model compound, and decarboxylation occurred primarily by an acid-promoted ionic mechanism that does not lead to cross-linking. However, pyrolysis in a nondonor solvent produced a small amount of products containing a new aryl-aryl bond between 3 and the solvent that represents the formation of a cross-link associated with the decarboxylation process. These cross-linked products were found to be formed by a free-radical pathway and could be decreased by the addition of H2O or tetralin to the pyrolysis medium. It is proposed that the cross-linked products arise from the formation and subsequent decomposition of anhydrides during the pyrolysis of the acid. Pyrolysis of di-3,3'-(2-(4-biphenyl)ethyl)benzoic anhydride and 1-(3-carboxaldehydephenyl)-2-(4-biphenyl)ethane was investigated to support the proposed free-radical formation of the cross-linked products. These results suggest that cross-linking processes in low-rank coals may not be directly related to the decarboxylation process but may indirectly result from intermediates formed from reactions of the aromatic carboxylic acids.