Tailoring the Subnano Silica Structure via Fluorine Doping for Development of Highly Permeable CO 2 Separation Membranes

The fluorine doping of a silica matrix was proposed in the design of enlarged and uniform networks for highly permeable CO 2 /CH 4 separation membranes. Fluorine doping effectively created a tunable network size, and the order of pore sizes increased with increases in the F concentration: F−SiO 2 (F/Si=2/8)>F−SiO 2 (F/Si=1/9)>SiO 2 . F−SiO 2 (F/Si=1/9) membranes showed high CO 2 permeance (4.1×10 −7  mol m −2  s −1  Pa −1 ) with high CO 2 /CH 4 selectivity (≈300) at 35 °C, due to the enlarged and uniform networks compared with those of SiO 2 . Fluorine doping was effective in preventing the formation of a 2nd Si−O in the 4‐membered ring (MR) of the SiO 4 tetrahedra. F−SiO 2 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.