Steric Hindrance Dominates the Systematic Substituent Effects of Pore-Dividing Agents on SF6/N2 Adsorption and Separation in PACS-MOFs

Pore-space-partitioned metal-organic frameworks (PSP-MOFs) offer a modular platform for gas adsorption, yet the influence of substituents on the pore-dividing agent remains unexplored. Herein, we report the first systematic investigation of substituent effects on 1,3,5-tri(pyridin-4-yl)benzene (TPB)-based pore-dividing ligands in four isostructural partitioned ACS MOFs (PACS-MOFs). By introducing substituents of increasing steric bulk (-H, -NH2, -CH3, and -OCH3) onto a fixed framework, we observe a clear, volume-dependent trend: SF6 uptake, SF6/N2 selectivity, and dynamic breakthrough separation time all follow the order unsubstituted > -NH2 ≈ -CH3 > -OCH3. Notably, the isosteric heats of adsorption for SF6 are remarkably similar across all four MOFs (21.18-26.15 kJ mol-1), confirming that the substituents exert negligible influence on the intrinsic binding sites. The performance differences are therefore dominated by steric hindrance that reduces pore accessibility, rather than by changes in binding affinity. This work establishes a fundamental design principle for PACS-MOFs: substituent volume on pore-dividing agents should be minimized to preserve pore accessibility for optimal separation performance of SF6.