Influence of adsorption on the diffusion selectivity for mixture permeation across mesoporous membranes

Molecular dynamics (MD) simulations were carried out to determine the self-diffusivities, D1,self, and D2,self for a variety of binary mixtures: methane (C1)–ethane (C2), C1–propane (C3), C1–n-butane (nC4), C1–n-hexane (nC6), C2–nC4, C2–nC6, Ar–C1, Ar–C2, Ar–C3, Ar–nC4, Ar–nC6, and Ar - Kr in a cylindrical silica mesopores. The diffusion selectivity, defined by (D1,self/D2,self) was found to be significantly different from the Knudsen selectivity, M2/M1, where Mi is the molar mass of species i  . For mixtures in which component 2 is more strongly adsorbed than component 1, (D1,self/D2,self)/M2/M1 has values in the range 1.5–4; the departures from the Knudsen selectivity increased with increasing differences in adsorption strengths of the constituent species.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (101 K)Download as PowerPoint slideResearch highlights▶ For molecules that adsorb strongly at the mesopore walls, the Knudsen diffusivity formula does not hold. ▶ The stronger the adsorption, the lower is the diffusivity prescribed by Knudsen. ▶ For binary mixtures, the diffusion selectivity can be significantly different from the Knudsen selectivity.