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'-di carboxyphenyl)ethane (2) has been investigated between 325 and 425 deg rees C as models of free carboxylic acids in low-rank coal to determin e if decarboxylation contributes to cross-linking during the thermal p rocessing of low-rank coals. The major reaction pathway from the pyrol ysis of 1 and 2 was decarboxylation to pro duce 1-(3-carboxyphenyl)-2- phenylethane and 1-(4-carboxyphenyl)-2-phenylethane. No evidence for a ryl-aryl coupling (cross-linking) or high molecular weight products wa s found, and mass balances were excellent (>97%) even at high conversi ons (up to 67%). The apparent first-order rate constant for the decarb oxylation of 1 to 1-(3-carboxyphenyl)-2-phenylethane was found to be l og k (s(-1)) = (9.4 +/- 0.4)-(43 +/- 1)/2.303RT (kcal/mol). It is prop osed that the thermal decarboxylation of 1 and 2 proceeds by an acid-p romoted, ionic mechanism. The involvement of a free radical decarboxyl ation pathway was eliminated by the pyrolysis of ,2-tetradeuterio-1,2- (3,3'-dicarboxyphenyl)ethane. These results raise into question whethe r decarboxylation and CO2 evolution are directly related to cross-link ing reactions in low-rank coals.