Balancing Fluorine Density and Pore Architecture in Nanoporous Polyketaminal Networks for Exceptional SF 6 /N 2 Separation

Abstract

Fluorination is a widely adopted strategy to enhance SF6 capture in porous adsorbents. However, the interplay between fluorine density and pore architecture remains insufficiently understood. In this work, we synthesized two fluorinated polyketaminal networks (PKANs) with distinct fluorine contents to investigate the trade-offs between chemical affinity and steric effects. The moderately fluorinated PKAN-2 (4.2 wt% F) exhibits an exceptional SF6/N2 IAST selectivity of 150.1 at 298 K, significantly outperforming the more heavily fluorinated PKAN-3 (13 wt% F). Although maximizing fluorine content is a common design strategy, our findings reveal that excessive functionalization increases the steric strain and torsional distortion of the monomers. This hinders efficient π–π stacking and framework densification, leading to the formation of textural mesopores (~3.9 nm) and a decreased specific surface area. Consequently, PKAN-2 preserves well-defined ultramicropores (~0.6 nm) that provide optimal spatial confinement, optimizing host-guest interactions through pore-wall potential overlap, whereas PKAN-3 suffers from a diminished confinement effect due to steric-induced packing defects. This study establishes that for fluorinated adsorbents, optimizing the microenvironment is more critical than simply maximizing the functional group density.