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.