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

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 hydrogenbonded 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 center dot 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 center dot center dot center dot H-O > O-H center dot center dot center dot O-C > O-H center dot center dot center dot O = C. Overall, this attractive strategy provides ideas for efficient separation of CAs, which will pave the way for utilization of lignite.

Automated summary

In this study, systematic processes were constructed to separate valuable carboxylic acids from lignite. Through distillation, ether extraction, acetone dissolution, and separation by deep eutectic solvent, the target carboxylic acids were successfully separated. The results showed that choline chloride has better separation performance, and the hydrogen bonds formed in the deep eutectic solvent can enhance separation efficiency.