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

Area of Science:

• Materials Science
• Chemical Engineering
• Separation Science

Background:

• Propylene/propane mixtures present a separation challenge due to similar molecular properties.
• Metal-organic frameworks (MOFs), like zeolitic imidazolate framework-8 (ZIF-8), offer tunable pore sizes for separation.
• Molecular-level modifications in MOFs for enhanced separation are difficult to achieve.

Purpose of the Study:

• To develop a method for reversibly tuning propylene/propane adsorption selectivity in ZIF-8.
• To investigate the influence of particle size and gas flow rate on separation performance.
• To introduce a new concept, dynamic selectivity, for understanding separation mechanisms.

Main Methods:

• Systematic variation of ZIF-8 crystal sizes (9–224 μm).
• Manipulation of propylene/propane gas flow rates during adsorption experiments.
• Development and application of a driving force equation based on dynamic selectivity for computational simulations.

Main Results:

• Increasing ZIF-8 crystal size reversed propane's preferential adsorption over propylene by restricting propane diffusion.
• Higher gas flow rates shifted the separation process from thermodynamic equilibrium to kinetics, reversing adsorption preference.
• The proposed dynamic selectivity concept successfully explained these reversed adsorption behaviors.

Conclusions:

• Diffusion-based modulation via particle size and gas flow rate is a viable strategy for optimizing separation in ZIF-8.
• Dynamic selectivity provides a framework for understanding and predicting separation performance under varying conditions.
• This approach offers a practical method for enhancing separation efficiency in porous materials without complex molecular modifications.