Functionalized UiO-66-NO2 for selective separation of coal-derived C8 cyclohexane isomers: A dual mechanism of geometric confinement and electronic recognition

The separation of coal-derived C8 cyclohexane isomers, particularly cis-1,3-dimethylcyclohexane (cis-1,3-DMCH) and ethylcyclohexane (ECH), remains challenging due to their similar molecular structures and physicochemical properties, while research on cycloalkane isomer separation is limited. A series of functionalized UiO-66 derivatives (UiO-66, UiO-66-(CH 3 ) 2 , UiO-66-OH, and UiO-66-NO 2 ) were synthesized via solvothermal methods and systematically evaluated for their adsorption and separation performance toward C8 cyclohexane isomers through single-component and competitive batch adsorption experiments, dynamic breakthrough tests, and molecular simulations. Among the investigated materials, UiO-66-NO 2 demonstrated superior selectivity (α = 5.10) for cis-1,3-DMCH over ECH, with excellent regeneration stability over multiple cycles, significantly outperforming commercial activated carbons and zeolites. Mechanistic studies revealed that the enhanced selectivity arises from a dual mechanism combining geometric confinement effects and electronic recognition, where the oblate geometry and larger dipole moment of cis-1,3-DMCH enable preferential adsorption through strengthened C-H···O and C-H···NO 2 interactions, as confirmed by calculated binding energies (−37.15 kJ/mol for cis-1,3-DMCH vs. -22.13 kJ/mol for ECH) and diffusion coefficients. This work demonstrates rational MOF functionalization for efficient cycloalkane isomer separation, providing a promising strategy for coal-derived naphtha valorization.