A Dual Locking Strategy Enables Highly Selective meta-Xylene Separation With an Aluminum-Pyrrolic Framework.

Efficient separation of xylene isomers remains a significant challenge due to their nearly identical physicochemical properties. Here, we report a dual locking strategy in an aluminum-pyrrolic framework (Al-PyDC) with V-shaped ligand featured AlO6 chains and pyrrole dual recognition sites that simultaneously stabilize the V-shaped meta-xylene (mX). This dual-locking effect promotes dense packing of mX within confined channels, resulting in a high adsorption capacity for mX (4.0 mmol g-1), which is much higher than that for ortho-xylene (oX, 2.5 mmol g-1) and para-xylene (pX, 2.0 mmol g-1). The simulations and solid-state NMR spectroscopy further confirmed that mX experienced stronger locking interactions with the AlO6 chains and pyrrolic ligands compared to pX. Notably, the dual locking effect was temperature-dependent, and elevated temperature accelerated molecular diffusion, thereby weakening the effectiveness of the recognition. Hence, Al-PyDC achieved an excellent mX/pX selectivity of 8.0 in vapor-phase breakthrough measurements at ambient temperature (303 K), which outperformed most of the MOF counterparts. Additionally, the isomorphic CAU-10-H with V-shaped AlO6-phenyl-AlO6 dual locking sites was employed as a control material to elucidate the role of active-site in molecular recognition. This dual locking strategy offered an effective approach for enhancing the adsorption capacity and selectivity for challenging isomer separations.