Systematic separation of carboxylic acids from H8[PMo7V5O40]-catalyzed lignite oxidation products via deep eutectic solvent

Abstract

To separate the valuable carboxylic acids (CAs) from the complicated oxidized products (OPs) of H8[PMo7V5O40]-catalyzed lignite, systematic processes including distillation, ether extraction, acetone dissolution and separation by deep eutectic solvent (DES) were first constructed and sequentially performed. First, formic acid and acetic acid are separated in vacuum at 56 °C with the separation efficiency of 95% and 82%, respectively. Subsequently, sequential treatments of OPs by ether extraction and acetone/water dissolution are implemented to reduce the low-polarity compounds contents and fully dissolve CAs from concentrated residue. Separation results indicate that choline chloride (ChCl) shows a better separation performance compared to acetylcholine chloride and gives optimal separation efficiencies of 78.19% and 94.45% for benzenepentacarboxylic acid (BPA) and mellitic acid (MLA) within 2 h at ambient temperature, respectively. The hydrogen-bonded binding energies (BEs) formed in DES are calculated based on density functional theory, and results show that total BEs of MLA and ChCl is as high as −31.09 kcal/mol, and the next is −26.79 kcal/mol for BPA, leading to their quick and high separation efficiencies within 2 h. As a result, only MLA and BPA show the high correlation with pseudo-second-order kinetic model, and the rate constant of MLA is as high as 0.3367 g/(mg·h), which is far more than that of BPA. Additionally, the effect of hydrogen bonds on the separation of CAs follows the order of Cl∙∙∙H-O > O-H∙∙∙O-C > O-H∙∙∙O = C. Overall, this attractive strategy provides ideas for efficient separation of CAs, which will pave the way for utilization of lignite.