(99e) Revisiting the Solution-Diffusion Model for Gas Permeation in Membranes

Membrane-based gas separation relies on the selective permeation of gas molecules through membrane materials, which can include polymers, zeolites, metal-organic frameworks (MOFs), or other ultramicroporous materials (pore size < 0.7 nm). The most widely adopted framework for describing this process is the solution-diffusion model, which divides gas transport into three steps: adsorption from the gas phase into the membrane, diffusion within the membrane, and desorption from the membrane into the downstream gas phase. According to this model, gas permeability is calculated as the product of diffusivity and solubility. However, this approach can be significantly misleading, as it neglects the kinetics of adsorption and desorption, which may play a critical role in determining overall permeation rates. In this study, we present a microkinetic modeling approach that explicitly accounts for the adsorption and desorption kinetics at the gas–membrane interfaces. This methodology enables a more accurate quantification of gas permeation by incorporating interfacial transport processes in addition to bulk diffusion. We demonstrate a quantitative comparison between the traditional solution-diffusion model and our proposed kinetic model, revealing notable discrepancies. By introducing additional parameters to describe interfacial transport, our model allows for a more precise calculation of gas permeability based on intrinsic adsorption and diffusion properties. This new methodlogy is broadly applicable to various membrane systems and offers enhanced predictive capability for gas separation performance.