Leveraging Diffusion Kinetics to Reverse Propane/Propylene Adsorption in Zeolitic Imidazolate Framework-8.

The separation challenge posed by propylene/propane mixtures arises from their nearly identical molecular sizes and physicochemical properties. Metal-organic frameworks (MOFs) have demonstrated potential in addressing this challenge through the precision tailoring of pore sizes and surface chemistry. However, introducing modifications at the molecular level remains a considerable hurdle. This work presents an approach to reversibly tune the propylene/propane adsorption preference in zeolitic imidazolate framework-8 (ZIF-8) by manipulating the particle size and gas flow rate. Systematically increasing the ZIF-8 crystals from 9 to 224 μm restricts propane diffusion, thereby reversing its preferential adsorption over propylene. Furthermore, raising the gas flow rate of mixed propylene/propane shifts the rate-determining breakthrough step from thermodynamic equilibrium to kinetics, again reversing the adsorption preference in a particular ZIF-8 sample. We propose "dynamic selectivity (Sd(t))" as a concept that incorporates both thermodynamic and kinetic factors to elucidate these unexpected findings. Moreover, the driving force equation, grounded on the concept of Sd(t), has improved the precision and stability of the computational simulation for fixed-bed adsorption processes. This work underscores the potential of diffusion-based modulation, implemented through manageable external changes, as a viable strategy to optimize separation performance in porous adsorbent materials.