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 hydr ogen-donor and nondonor solvents to determine the role of decarboxylat ion of aromatic carboxylic acids in the cross-linking processes in low -rank coal. Decarboxylation was the dominant reaction pathway in the p yrolysis of this model compound, and decarboxylation occurred primaril y by an acid-promoted ionic mechanism that does not lead to cross-link ing. 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 deca rboxylation process. These cross-linked products were found to be form ed 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 cros s-linked products arise from the formation and subsequent decompositio n 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-ra dical formation of the cross-linked products. These results suggest th at cross-linking processes in low-rank coals may not be directly relat ed to the decarboxylation process but may indirectly result from inter mediates formed from reactions of the aromatic carboxylic acids.