Occupancy Dependency of Maxwell-Stefan Diffusivities in Ordered Crystalline Microporous Materials

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Diffusion in porous materials like zeolites and metal-organic frameworks (MOFs) is crucial for separation processes.
• Maxwell-Stefan (M-S) diffusivities are key parameters but their strong dependence on molar loadings complicates analysis.
• Existing models struggle to accurately capture the complex interplay of guest molecules within these materials.

Purpose of the Study:

• To establish a fundamental framework for understanding and describing the loading dependence of M-S diffusivities in binary mixtures within porous materials.
• To introduce a thermodynamically rigorous metric, occupancy (θ), as a reliable proxy for spreading pressure (π) to characterize diffusion behavior.
• To provide a simplified interpretation of diffusion in mixtures using lattice models.

Main Methods:

• Utilized molecular dynamics simulations to generate M-S diffusivity data for various binary guest mixtures in zeolites and MOFs.
• Employed the ideal adsorbed solution theory to derive a thermodynamically sound definition of occupancy (θ).
• Performed configurational-bias Monte Carlo simulations to calculate spreading pressure (π) and occupancy (θ) from unary adsorption isotherms.

Main Results:

• Demonstrated that M-S diffusivities (Đ) are strongly dependent on molar loadings.
• Showed that M-S diffusivities for a component in a binary mixture are equivalent to its unary diffusivity when compared at the same occupancy (θ).
• Found that the M-S diffusivity of a component in a mixture is independent of its co-diffusing species at constant θ.
• Observed a linear increase in the ratio Đ₁/Đ₁₂ (degree of correlations) with occupancy (θ), indicating increased importance of correlations near pore saturation.

Conclusions:

• Occupancy (θ) provides a robust and simplified metric for describing the loading dependence of M-S diffusivities in porous materials.
• The findings offer a more fundamental understanding of diffusion mechanisms in complex guest-host systems.
• This work facilitates more accurate predictions and design of separation processes utilizing zeolites and MOFs.