Reaction of carboxylic acids under coal liquefaction conditions - 1. Under nitrogen atmosphere

The pyrolytic reactions of benzoic, p-hydroxybenzoic, and 2-naphthoic acid were examined in microautoclave reactors at 350-450 degrees C in nitrogen. The temperature conditions are similar to those used for the direct liquefaction conversion of coals to synthetic liquid fuels. Reactions of salts of the acids were also examined, since many acid groups in coals are present as a salt. In addition, reactions were also examined in the presence of water, the role of which in liquefaction is controversial, and in the presence of pyridine, a solvent used for extractions or swelling of the coal prior to liquefaction. A general reaction pathway for the benzoic and 2-naphthoic acids involves anhydride formation in the absence of water. The anhydride further decomposes to yield decarboxylation products, dimers and polymers. Dimer or polymer formation is undesirable in coal conversion processes. Decarboxylation proceeding through an electrophilic mechanism by H+ attack on the ipso- carbon, which is the desired route, is a competing pathway, p-Hydroxybenzoic acid predominantly decarboxylates via the latter route. p-Hydroxybenzoic acid tends to polymerize at low temperatures in the absence of water. Pyridine catalyzes decarboxylation of aromatic acids. Calcium and sodium salts of aromatics showed very low reactivity to pyrolysis under these conditions. The mechanism of conversion in these systems is not unique, but rather depends on the structure of the acid and its reaction conditions (e.g. temperature and other species present in the system). This shows that one could alter the course of pyrolysis of these compounds by appropriate choice of reaction conditions. It also shows that pyrolysis of a complex material like coal, in which a variety of structures and functional groups are present, likely involves numerous reaction pathways even at a single set of reaction conditions. (C) 2000 Elsevier Science B.V. All rights reserved.