Crucial role of sulfonated ionic liquids as promoter for efficient production of valuable carboxylic acids by H5PMo10V2O40-catalyzed aerobic oxidation of lignite

Abstract

To explore the effect of adding acidic ionic liquids (ILs) to the heteropoly acid/H2SO4 catalytic system on the aerobic oxidation of lignite, acidic ILs functionalized with -SO3H were synthesized accordingly and mixed with H5PMo10V2O40 (HPA-2) in various molar ratios. The optimum combination of HPA-2 and 1-methyl-3-(4-sulfobutyl) imidazolium (MIMBS) with molar ratio of 0.3 showed excellent performance for the catalytic oxidation of lignite, with the yields of carboxylic acids (CAs) (including benzene carboxylic acids, formic acid and acetic acid) reaching 326.21 ± 12.86 mg/g lignite, which considerably exceeded the 136.08 ± 6.23 mg/g lignite of pure HPA-2. Highest occupied molecular orbital energy of CAs calculated by Gaussian 09 on 6-31G(d) basis set and Hammett function analysis confirmed that the oxidative activity and acidity of the catalytic system could be moderately modulated by sulfonated ILs, thereby increasing the yield of CAs. In addition, the effect of H2SO4 concentration on the oxidation of lignite was investigated via the HPA-2/H2SO4/MIMBS system with different molar ratios, and the results showed that CAs yield increased first and then decreased with increasing H2SO4 concentration, but the concentration of H2SO4 at the maximum yield of CAs was significantly reduced from 0.28 wt% to 0.14 wt% once MIMBS was added. A possible process for the catalytic oxidation of lignite by sulfonated ILs/HPA-2/H2SO4 in aerobic aqueous solution is proposed, which generally involves the reactions of hydrolysis, oxidation and re-oxidation. Due to the good reproducibility, the robust homogeneous catalytic system of HPA-2/sulphonated ILs has the advantages of including increased yields of CAs and reduced H2SO4 dosage, which will pave the way for the efficient utilization of lignite.