Does Decarboxylation Lead to Cross-Linking in Low-Rank Coals?

The pyrolysis of 1,2-(3,3‘-dicarboxyphenyl)ethane ( 1 ) and 1,2-(4,4‘-dicarboxyphenyl)ethane ( 2 ) has been investigated between 325 and 425 °C as models of free carboxylic acids in low-rank coal to determine if decarboxylation contributes to cross-linking during the thermal processing of low-rank coals. The major reaction pathway from the pyrolysis of 1 and 2 was decarboxylation to produce 1-(3-carboxyphenyl)-2-phenylethane and 1-(4-carboxyphenyl)-2-phenylethane. No evidence for aryl−aryl coupling (cross-linking) or high molecular weight products was found, and mass balances were excellent (>97%) even at high conversions (up to 67%). The apparent first-order rate constant for the decarboxylation of 1 to 1-(3-carboxyphenyl)-2-phenylethane was found to be log k (s - 1 ) = (9.4 ± 0.4)−(43 ± 1)/2.303 RT (kcal/mol). It is proposed that the thermal decarboxylation of 1 and 2 proceeds by an acid-promoted, ionic mechanism. The involvement of a free radical decarboxylation pathway was eliminated by the pyrolysis of 1,1,2,2-tetradeuterio-1,2-(3,3‘-dicarboxyphenyl)ethane. These results raise into question whether decarboxylation and CO 2 evolution are directly related to cross-linking reactions in low-rank coals.