Acidity modulation mechanisms governing selective extraction of nitrogen heterocycles in dual-acid DESs

Selective capture of structurally diverse molecules remains a major challenge in chemical separations. Here, we introduce dual-acidic deep eutectic solvents (DESs) in which Lewis acids modulate the Brønsted acidity, creating a synergistic environment for the simultaneous extraction of basic and non-basic nitrogen-containing molecules. Brønsted sites, activated by adjacent Lewis centers, preferentially bind basic heterocycles, while the Lewis sites coordinate with non-basic species, achieving unprecedented extraction efficiencies. Mechanistic studies combining FT-IR spectroscopy, density functional theory, and molecular electrostatic potential analysis reveal that proton transfer, Lewis acid–base coordination, and π–cation interactions collectively govern selective recognition. This dual-acidic design strategy is generalizable across different LA-metal DES compositions and robust against competing chemical species. Our findings demonstrate a rational approach to fine-tune cooperative acid functionalities, establishing a versatile platform for selective molecular capture and separations beyond conventional single-acid systems.