Production of Benzene Polycarboxylic Acids from Lignite by Alkali-Oxygen Oxidation

The oxidation of coal to produce high-valued benzene polycarboxylic acids (BPCAs), which are obtained currently from diminishing petroleum reserves, is a promising industrial process of the future. Up to now, the yield distribution of BPCAs has not been studied in detail and the mechanism of coal oxidation to BPCAs remains unclear. In this study, Huolinhe lignite was oxidized in a batch reactor by alkali-oxygen oxidation. All 12 kinds of BPCAs obtained were quantified by a new established method. Effects of alkali/coal mass ratio, reaction temperature, initial oxygen pressure, and reaction time on the yield distribution of BPCAs were studied for the first time The results indicate that BPCAs with four or five carboxyls are the predominant products, and BPCAs with one or two carboxyls are formed in a relatively short time; moreover, the formation of BPCAs with more carboxyls is relatively more sensitive to the salting out effect. CP/MAS C-13 NMR spectra and oxidation of model compounds show that phenolic, ether-substituted aromatic, ether, and aldehyde groups are easily converted and that water-soluble acids (WSA) are formed rapidly and largely due to the breakage of these bonds. The step from WSA to BPCAs is relatively slow mainly due to the inertia of the aromatic clusters with attached carboxyls or carboxylate. On the whole, the BPCAs are derived from aromatic clusters through the oxidation of condensed benzene rings, bridges, or peripheral groups that are attached to the aromatic clusters. Possible mother units for BPCAs in the lignite are suggested based on the generally agreed lignite structure.