Catalytic O2 oxidation of lignite to carboxylic acids with iron-based catalysts in acidic aqueous solutions

Abstract

Carboxylic acids (CAs) are important industrial materials. The properties of lignite determine that it has great potential to produce CAs by oxidation. In this work, an environmentally benign catalyst, Fe2(SO4)3, was used to oxidize lignite by O2 in acidic aqueous solutions to produce CAs. The effects of Fe2(SO4)3 concentration, H2SO4 concentration, reaction temperature, reaction time and initial O2 pressure on lignite conversion and CAs yield were studied. We have found that the catalytic system can effectively promote the oxidation of lignite. H2SO4 could adjust the activity of Fe2(SO4)3, which influences the yields of formic acid and benzene carboxylic acids (BCAs) greatly. A CAs yield of 48.3 wt% was obtained, including 21.6 wt% formic acid, 12.7 wt% acetic acid, 0.9 wt% succinic acid, 4.2 wt% oxalic acid and 8.9 wt% BCAs. The catalytic system can be reused after the extraction of products. Moreover, FeS2 is readily available as the main component of pyrite that is an inorganic mineral of coal. Thus, FeS2 was also studied for the oxidation of lignite. The catalytic system of FeS2/H2SO4 achieves the same CAs yield as Fe2(SO4)3/H2SO4 with the same amounts of iron and sulfur. In the oxidation conditions, iron and sulfur of FeS2 were converted to Fe3+ and SO42−, respectively. The catalysts developed in this work are non-corrosive, easy to obtain and environmentally friendly.