Simultaneous production of small-molecule fatty acids and benzene polycarboxylic acids from lignite by alkali-oxygen oxidation

Alkali-oxygen oxidation of coals to produce high-value benzene polycarboxylic acids (BPCAs) is a potential route in the future. Among the coal ranks, lignite has been proved to be easily oxidized due to its low coalification, but the yield of BPCAs obtained from lignite is relatively low because of the low content of aromatic clusters in its structures. However, the aliphatic structures of lignite are relatively rich, which may be served as the mother structures for small-molecule fatty acids (SMFAs). Thus, in the present work, three lignites were oxidized in aqueous alkaline solutions by oxygen. SMFAs were detected and quantified in addition to BPCAs after the reaction. The results show that when the optimal BPCAs yields (18.4–21.5%) are obtained, large amount of SMFAs (39.8–23.2 wt.%) can be obtained simultaneously, including oxalic acid, formic acid, acetic acid, succinic acid and malonic acid. The oxidation of model compounds shows that SMFAs are mainly derived from the oxidation of aliphatic structures as well as the opening of benzene rings in the lignite structure.

► Up to 58% yields of fatty acids and benzene polycarboxylic acids by lignite oxidation ► Fatty acids are derived from aliphatic structures and opening of benzene rings. ► An efficient way has been proposed to convert lignite to valuable chemicals.