Efficient separation of indole from fossil fuel pyrolysis products by carboxylic acid non-aromatic ring ionic liquids: Experiment and mechanism exploration

Abstract

Indole is a nitrogen-containing compound that is obtained from fossil fuel pyrolysis products. Its usage is not only related to the production of fine chemicals and medicines, but it also has the potential to benefit environmental protection. To separate indole through liquid–liquid extraction with ionic liquids, two types of carboxylic acid non-aromatic ring ionic liquids (CNILs) were successfully synthesized and characterized in this work. The effects of stirring time, mCNILs: mmixtures, and extraction temperature were extensively investigated in this work. The results indicate that the adopted CNIL, [N-tmg][Bpc], which processes multifunctional sites, exhibited better extraction performance for indole. Under the optimized conditions (mCNILs: mmixtures = 1:5, temperature, 25 °C, stirring time, 30 min) and initial indole content, mindole: moil = 5:100, the extraction efficiency of indole was found to be 97.08% using the CNIL [N-tmg][Bpc]. Additionally, the molecular polarity, bond length, interaction energy, and non-covalent interaction were evaluated via quantum chemistry calculation, and found that the non-covalent interaction between the CNIL anions and indole was the primary driver of extraction, and the multifunctional sites in CNIL anions improved the ability of CNILs to extract indole. Finally, the reusability of CNIL, [N-tmg][Bpc], was tested through repeated extraction experiments, and the recovered [N-tmg][Bpc] was found to efficiently extract indole. The successful synthesis of CNILs with high extraction performance for indole provides a novel route to develop new ionic liquid separation agents. The finding in this work contributes to the chemical industry's efforts to develop appropriate and sustainable technologies for indole separation.