Relationship between Ethane and Ethylene Diffusion inside ZIF-11 Crystals Confined in Polymers to Form Mixed-Matrix...

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Mixed-matrix membranes (MMMs) incorporate porous materials within polymer matrices for enhanced separation properties.
• Zeolitic imidazolate frameworks (ZIFs), like ZIF-11, are promising materials for gas separations due to their tunable structures.
• Understanding gas diffusion within ZIFs confined in MMMs is crucial for membrane performance.

Purpose of the Study:

• To investigate the effect of polymer confinement on the self-diffusivity of ethane and ethylene within ZIF-11 crystals.
• To compare intracrystalline diffusion in ZIF-11 within MMMs versus ZIF-11 crystal beds.
• To explore the influence of polymer properties and ZIF-11/polymer adhesion on confinement effects.

Main Methods:

• Multinuclear pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) was used to measure self-diffusivities.
• Measurements were conducted on ZIF-11 crystals dispersed in Torlon, Matrimid, and 6FDA-DAM polymers.
• Temperature-dependent diffusion studies were performed to determine activation energies.

Main Results:

• Confinement of ZIF-11 in Torlon decreased ethane and ethylene self-diffusivity and increased diffusion activation energy.
• This decrease was attributed to reduced ZIF-11 framework flexibility and smaller effective aperture size.
• Intra-ZIF diffusion selectivity for ethane/ethylene remained largely unaffected by confinement in Torlon.
• No significant confinement effect on diffusivity was observed in ZIF-11/Matrimid and ZIF-11/6FDA-DAM MMMs.

Conclusions:

• Polymer confinement can alter gas diffusion within ZIFs, impacting membrane performance.
• The extent of the confinement effect is influenced by the polymer's bulk modulus and ZIF-11/polymer adhesion.
• ZIF-11/Torlon MMMs exhibit a confinement effect, while ZIF-11/Matrimid and ZIF-11/6FDA-DAM MMMs do not, suggesting polymer-specific interactions.