Selective separation of aromatic N -heterocyclic compounds by cyclodextrins based on the size-matching effect

Removing ANCHCs such as indole and carbazole from various raw materials is of great significance for the clean utilization of materials and the reduction of environmental pollution; it can also promote the sustainable exploitation of resources and realize ecological environment protection. In this study, natural α-, β-, and γ-cyclodextrins (CDs) were used as novel extractants to investigate the extraction rules in simulated oil affected by the CDs’ cavity size. Preliminary screening shows different CDs exhibited distinct extraction tendencies toward the target compounds, reflecting the influence of host-guest size matching. β-CD showed a preference for indole, while γ-CD preferred carbazole. The effects of temperature, time, and extractant-to-oil ratio were further investigated to determine suitable conditions. In mixed indole/carbazole, β- and γ-CD displayed moderate selectivity, rather than a complete one-step separation. FT-IR, XRD, and SEM analyses indicated changes in intermolecular interactions, crystalline arrangement, and morphology after complex formation. IGMH, AIM, geometrical inclusion analysis, and BSSE-corrected binding energies further revealed that the preferential association of indole with β-CD arises from more favorable cavity-size matching together with van der Waals, hydrophobic, and other weak attractive interactions. This study provides a model-system basis for understanding CD-assisted preferential extraction of structurally similar ANCHCs, and offers guidance for the further design of CD-based green extraction systems.