Tailoring the Subnano Silica Structure via Fluorine Doping for Development of Highly Permeable CO₂ Separation Membranes

Abstract The fluorine doping of a silica matrix was proposed in the design of enlarged and uniform networks for highly permeable CO₂/CH₄ separation membranes. Fluorine doping effectively created a tunable network size, and the order of pore sizes increased with increases in the F concentration: F−SiO₂ (F/Si=2/8)>F−SiO₂ (F/Si=1/9)>SiO₂. F−SiO₂ (F/Si=1/9) membranes showed high CO₂ permeance (4.1×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) with high CO₂/CH₄ selectivity (≈300) at 35 °C, due to the enlarged and uniform networks compared with those of SiO₂. Fluorine doping was effective in preventing the formation of a 2nd Si−O in the 4‐membered ring (MR) of the SiO₄ tetrahedra. F−SiO₂ samples with smaller fractions of 4 MR and a smaller ring in silica, contain large fractions of larger rings (>5 MR), resulting in enlarged network structure and an increase in the average pore size, which translates to a higher permeance for larger molecules.