97/01507 Decarboxylation of coal model compounds under liquefaction conditions: Does decarboxylation lead to retrograde reactions?

Abstract

In recent years, it has become clear that oxygen functional groups in low-rank coals are major actors in retrograde reactions which inhibit their efficient thermochemical processing. In the pyrolysis and liquefaction of low-rank coals, low-temperature cross-linking reactions have been correlated with the loss of carboxyl groups and the evolution of CO{sub 2} and H{sub 2}O. Pretreatments such as methylation, demineralization, or ion-exchange of the inorganic cations reduce cross-linking and CO{sub 2} evolution in pyrolysis. Cross-linking reactions also have a deleterious effect on liquefaction yields and the distribution of oils, preasphaltenes and asphaltenes. These results suggest that decarboxylation may occur by a pathway that initiates retrograde (cross-linking) reactions in the coal polymer independent of the reaction conditions. However, the decarboxylation pathways in liquefaction and pyrolysis of low-rank coals are not known, and it is not clear how decarboxylation leads to cross-linking. Radical recombination or radical addition reactions have been suggested as being involved in retrograde reactions. However, the involvement of radical pathways in thermal decarboxylation reactions has recently been brought into question. We have presented evidence that in the pyrolysis of several bibenzyls containing aromatic carboxylic acids, radical pathways are not involved in thermal decarboxylation reactions and no cross-linking or couplingmore » products are formed. Further, Manion et al. observed that decarboxylation of benzoic acid derivatives in tetralin yielded only small amounts of aryl-aryl coupling products. To gain a better understanding of the role decarboxylation plays in cross-linking reactions during liquefaction in low-rank coals, we have studied the thermal decomposition of several bibenzyls containing aromatic carboxylic acids, and their salts, in the presence of a hydrogen donor solvent (tetrahn) and a nondonor solvent (naphthalene).« less