Experimental and quantum chemical calculations investigations of morpholine-based ionic liquids as extractants for efficient extraction of nitrogen heterocyclic neutral compounds

Indole and pyrrole that are present in coal tar and fuel oil are representatives of nitrogen heterocyclic neutral compounds (NHNCs). On the one hand, the presence of NHNCs in the hydrodenitrogenation (HDN) process deactivated the catalyst while causing harm to the environment. On the other hand, NHNCs were irreplaceable basic raw materials and intermediates in industry. Therefore, the separation of NHNCs from oil was necessary. In this work, morpholine-based ionic liquids (Mor-based ILs) with low toxicity and biodegradability were consid-ered as green extractants for NHNCs, three Mor-based ILs 1-methyl morpholine p-toluenesulfonate ([C1Mor] [TOS]), 1-methyl morpholine benzoate ([C1Mor][BEN]) and 1-butyl-1-methyl morpholinium thiocyanate ([C1C4Mor][SCN]) were synthesized and then characterized by 1H NMR. All of them were applied to the extraction of NHNCs, and their extraction performance was evaluated by extraction efficiency (EE). The order of the EE for NHNCs was [C1C4Mor][SCN] > [C1Mor][TOS] > [C1Mor][BEN]. Based on this, the extraction con-ditions of the promising [C1C4Mor][SCN] were optimized, and the EE was 96.89 % and 95.62 % for indole and pyrrole, respectively. Moreover, quantum chemical (QC) calculations were adopted to evaluate the extraction performance at the microscopic nature of the differences, and the theoretical analysis and experiments agreed well with each other. Then the mechanism of NHNCs extraction by [C1C4Mor][SCN] was explored at the mo-lecular level based on density functional theory (DFT). The results showed that N-HMIDLINE HORIZONTAL ELLIPSISN hydrogen bonding interaction was formed between anion and NHNCs, while the cation interacted with NHNCs as van der Waals interaction. Finally, the reusability of IL was investigated.

Automated summary

This study used morpholine-based ionic liquids as green extractants for NHNCs. It was found that 1-butyl-1-methyl morpholinium thiocyanate showed the best extraction performance. The mechanism of extraction between the ionic liquid and NHNCs was explored using density functional theory.