Controllable selectivity of carboxylic acids produced by H3+n[PMo12-nVnO40]-catalyzed oxidation of lignite and its oxidative mechanism analysis

Graphical This attractive work examines the changes in CAs selectivity when lignite is oxidized by H 3+n [PMo 12-n V n O 40 ] (HPA-n, n=1-5) and discloses the catalytic mechanism of HPA. The catalytic process of HPA follows the electron transfer-oxygen transfer (ET-OT) mechanism, where it not only activates lignite through electron transfer, but also activates oxygen, leading to the generation of active substance ⋅OH in water. The ⋅OH promotes the breakage of lignite. Although the oxidation of lignite has been extensively studied, the factors affecting the selectivity of carboxylic acids (CAs) are unclear. This attractive work examines the changes in CAs selectivity when lignite is oxidized by H 3+n [PMo 12-n V n O 40 ] (HPA-n, n=1-5) and discloses the catalytic mechanism of HPA. The results show that the selectivity of benzene polycarboxylic acids and acetic acid (AA) increases with increasing vanadium content of HPA-n, such as mellitic acid and benzenepentacarboxylic acid from 0.8 % and 3.0 % to 5.4 % and 16.2 %, respectively. Consequently, the differences in the concentration of HPA or H 2 SO 4 used to obtain optimal formic acid and AA yields confirm that the intrinsic properties of CAs calculated by Gaussian 09 can also affect their selectivity. HPA can not only activate lignite through electron transfer, but also further activate oxygen, leading to the generation of active substance ⋅OH in water. Consequentially, ⋅OH as the key reactive substance can promote the breakage of bridge bonds, side chains and condensed aromatic rings during HPA-catalyzed oxidation of lignite. Thus, the oxidation pathway of lignite over HPA is proposed. This work constructs a relationship between oxidation capacity and CAs selectivity and discloses the lignite oxidation pathway at the molecular level, which is a significant progress in HPA-catalyzed conversion of lignite.