Modulating Adsorption Kinetics in a 3D-Interconnected Nanocavity Framework with Narrow Apertures for Enhanced Propylene Separation

The energy-intensive distillation currently used for C₃H₆/C₃H₈ separation─challenged by their small boiling point difference─could be improved via adsorption. However, most porous materials face a trade-off among C₃H₆ adsorption capacity, selectivity, and kinetics. Herein, we report the synthesis and characterization of a novel metal-organic framework, denoted NiPz₄Bim, constructed from a weak Lewis-base pyrazole-based ligand Pz₄Bim and the weak Lewis-acid Ni²⁺, featuring 3D pore structures with nanocavities (∼1 nm) connected by very narrow apertures (∼5 Å). This framework enables efficient C₃H₆/C₃H₈ separation by combining selective adsorption with enhanced diffusion kinetics for C₃H₆. Specifically, adsorption capacities at 298 K and 1 bar were recorded as 3.24 mmol g^-1 for C₃H₆ and 2.74 mmol g^-1 for C₃H₈, with selectivity ratios of up to 2.42. Kinetic uptake analysis using the effective diffusion coefficient (D') revealed a significant difference in the adsorption rates of the two gases, corresponding to a kinetic selectivity of 51.96. Neutron powder diffraction, coupled with grand canonical Monte Carlo simulations and density functional theory calculations, directly visualizes the binding domains of adsorbed gases and the dynamics and energetics of the host-guest interactions. These studies reveal that the unique nanosized cavities with narrow apertures in NiPz₄Bim facilitates van der Waals and π-π interactions with C₃H₆, enabling selective trapping over C₃H₈. Crucially, NiPz₄Bim exhibits high stability and reusability in multicycle tests, demonstrating its practical viability. This work highlights the importance of pore-geometry engineering in framework materials for the efficient separation of structurally similar molecules, with immediate implications for sustainable olefin production.