Quantum chemical investigation of the thermal pyrolysis reactions of the carboxylic group in a brown coal model

Different reaction pathways of the carboxylic group in a brown coal model were investigated by applying density function quantum chemical theory, examining the possible cross-linking and decomposition reactions between the hydrogen bonded carboxylic group–carboxylic group and the carboxylic group–hydroxyl group during the thermal pyrolysis process. The results show that bimolecular dehydration and decarboxylation of hydrogen bonded carboxylic groups have distinctly lower activation barriers and therefore, proceed preferentially at low temperature. The esterification reaction between the hydrogen bonded carboxylic group and hydroxyl group, together with unimolecular decarboxylation of isolated single carboxylic groups were also possible at moderate temperature. Aryl–aryl coupling is thought to occur via radical pyrolysis and recombination at relatively high temperature.