Theoretical insight and experimental exploration of designing biocompatible functionalized ionic liquids for efficient separation of typical organic Lewis acid compound indole from coal-based fuel pyrolysis product

Organic Lewis acid compounds such as indole were compounds with high added value and mainly distributed in coal-based fuel pyrolysis product. Traditional indole acquisition methods inevitably produce pollutants without atomic economy, so it was extremely urgent to develop a greener and more sustainable strategy to separate indole. Then, the relatively mild liquid-liquid extraction method was selected. In order to reduce the environmental pollution and ecological toxicity of the extractants themselves, three biocompatible functionalized ionic liquids (ILs) were synthesized by one-step reaction and their structures were characterized. The probable cites on ILs were explored by sigma-profile and ESP analysis, pointing out that the presence of hydroxyl groups on cations and O atoms on anions showed higher polarities. And the intramolecular interactions of ILs can be enhanced by increasing the number of the hydroxyl group on cations. Moreover, the intramolecular interactions of ILs had an important impact on the intermolecular interactions between ILs and indole. The formed hydrogen bonds (H-bonds) and van der Waals interactions were confirmed as separation driving forces. Meanwhile, the viscosity of the ILs was analyzed. The factors affecting the extraction efficiency were explored and the extraction conditions optimized, which was consistent with the above theoretical analysis. This work sheds light on linking the intermolecular interactions between ILs and indole based on the detailed discussions of intramolecular interactions in ILs, which also provides an idea for the greener design of ILs to separate typical organic Lewis acid type compounds from coal-based fuel pyrolysis product. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study developed a greener and more sustainable strategy for separating indole from coal-based fuel pyrolysis products. By using a relatively mild liquid-liquid extraction method with biocompatible functionalized ionic liquids (ILs), indole was successfully separated. The intramolecular interactions of ILs were found to have a significant impact on the intermolecular interactions between ILs and indole.