Temperature-Responsive Xylene Isomer Separation via Biomimetic Window Confinement and Defect-Engineered Nanospace in Metal–Organic Frameworks

Biological channels achieve remarkable selectivity by amplifying subtle molecular differences through nanoscale confinement and steric gating. Inspired by this principle, we investigate the zirconium fumarate framework MOF-801, whose triangular pore windows act as biomimetic steric filters that preferentially accommodate para-xylene (PX) over its meta- (MX) and ortho- (OX) isomers. To further enhance the separation performance, we introduced structural defects that enlarge the internal nanospace while preserving the selective gating effect of the windows. The resulting material, MOF-801-132AA, delivers high liquid-phase separation performance, achieving a PX/OX selectivity of 103.9 at room temperature and a PX uptake of 243.8 mg g^-1, as determined from vapor adsorption measurements. MOF-801-132AA also exhibits a temperature-dependent selectivity inversion with the preferential order shifting from PX > MX > OX at 25-60 °C to OX > MX > PX above 160 °C. This unusual dynamic behavior is validated by liquid-batch adsorption, vapor-phase sorption, and breakthrough experiments. Single-crystal electron diffraction reveals preferential PX localization in the octahedral cages, while molecular simulations attribute the inversion to the interplay of window-blocking effects, isomer packing configurations, and defect-induced nanospace enlargement. Together, these results illustrate how defect-modulated nanospace and window confinement can be combined to enable the adsorption-based separation of structurally similar isomer mixtures.