Production of Benzene Carboxylic Acids and Small-Molecule Fatty Acids from Lignite by Two-Stage Alkali-Oxygen Oxidation

Carboxylic acids are widely used in industry as a kind of important chemical materials, and it is very promising to produce carboxylic acids from lignite. The traditional alkali-oxygen oxidation of lignite does not consider the complex structure of lignite, and the structures with different reactivities are treated under identical and harsh reaction conditions, resulting in a low yield of carboxylic acids. We propose a two-stage oxidation process of lignite to increase the yield and mitigate the harsh reaction conditions. First, lignite was extracted using a NaOH aqueous solution to yield two parts (extract and residue) with different reactivities. The extract and residue then were oxidized separately with alkali-oxygen oxidation under different conditions. In this way, high yields of benzene carboxylic acids (BCAs, 12 types) and small-molecule fatty acids (SMFAs) could be obtained. The extraction conditions and the alkali-oxygen oxidation conditions of the extract and the residue were investigated. Compared with the oxidation of extract, the oxidation of residue requires higher reaction temperature, higher initial O2 pressure, higher alkali concentration, and longer reaction time. The structures of the extract and residue were characterized by 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared (FT-IR) spectroscopy, which indicates that most of aromatic structures in the extract are single aromatic rings with side chains containing many oxygen-containing functional groups. However, aromatic structures in the residue are mainly condensed aromatic rings with side chains containing many alkyl chains. These differences in their structures result in different yields of carboxylic acids from the extract and residue.