Pyrolysis of aromatic carboxylic acid salts: Does decarboxylation play a role in cross-linking reactions?

The pyrolysis of sodium benzoate, potassium benzoate, and calcium benzoate has been investigated between 435 and 500 degrees C as models for carboxylic acid salts in low-rank coals to determine if the decarboxylation of benzoate salts can contribute to cross-linking reactions during the thermal processing of low-rank coals. Decarboxylation was the dominant reaction pathway for all three salts. However, char formation (leading to poor mass balance) was also observed in all cases. The reaction proceeds via an anionic pathway, where the initial decarboxylation of benzoate salt leads to the formation of a phenyl anion as the key intermediate. Proton abstraction by the phenyl anion from another molecule of benzoate leads to the formation of benzene and benzoate dianion, which can react with CO2 to form a di-salt of phthalic acid (i.e., the Henkel reaction). Both benzene and phthalate account for > 95% of the total products in the pyrolysis of sodium and potassium benzoates. The minor products diphenylmethane, benzophenone, triphenylmethane, and 9-phenylfluorene can form by a mechanism involving the reaction of phenyl anion with alkali benzoate to generate benzophenone, which reacts further to form the other products. Contrary to previous reports, calcium benzoate pyrolysis proceeds via an anionic mechanism to form benzene and benzophenone as the major products. No evidence of any calcium phthalate formation is observed in the pyrolysis of calcium benzoate. Furthermore, the reaction of benzophenone with the phenyl anion to form diphenylmethane, triphenylmethane, and 9-phenylfluorene seems to be slowed considerably (compared to sodium and potassium benzoates), based on the observed yield of these minor products. The rate constant for the pyrolysis of these benzoate salts, which decarboxylate slower than corresponding acids, decreases in the following order: potassium benzoate > sodium benzoate >> calcium benzoate. The pyrolysis rate is increased by at least a factor of 3 in the presence of water. Water traps the intermediate phenyl anion very efficiently to form benzene almost exclusively. Based on these findings, it can be concluded that the presence of water can drastically reduce cross-linking reactions and significantly improve liquefaction.