Countercation-Regulated Pore Structure Engineering of fcu-Metal-Organic Frameworks for Enhanced Gas Separation

Abstract

Metal–organic frameworks (MOFs) offer exceptional structural tunability for targeted gas adsorption and separation; however, traditional pore engineering via organic ligand functionalisation often necessitates complex synthetic routes. In contrast, substituting inorganic secondary building units (SBUs) offers a streamlined strategy for modulating the pore environment, particularly when introducing metals with varied valences that require charge-balancing counterions. Herein, we elucidate the regulatory role of extra-framework cations by conducting a comparative study between the anionic yttrium-based framework (Y-fum-fcu-MOF) and its neutral zirconium analogue (Zr-fum-fcu-MOF, MOF-801). Through high-resolution synchrotron X-ray powder diffraction and Rietveld refinement, we identify a unique ‘pincer-like’ coordination mechanism within the Y-MOF cavities, where CO2 molecules are synergistically stabilised by Y3+ centres and protonated dimethylammonium (DMA-H+) counteraction. This cooperative interaction effectively constrains the rotational and translational degrees of freedom of the guest molecules, resulting in a significantly enhanced isosteric heat of adsorption (Qst = 38 kJ mol-1). Consequently, Y-MOF exhibits a CO2 uptake of 85.11 cm3 g-1 at 298 K, representing a 46.4% increase over the neutral Zr-MOF, alongside superior CO2/N2 selectivity. These findings demonstrate that counteraction engineering can induce localised ‘electrostatic locking’ of guest molecules, providing a robust molecular-level blueprint for designing high-performance adsorbents for industrial carbon capture and gas separation.