Tailoring the structure of a sub-nano silica network via fluorine doping to enhance CO2 separation and evaluating CO2 separation performance under dry or wet conditions

The amorphous nature of a silica network structure offers an attractive opportunity for excellent separation of small molecules such as He and H2. CO2 separation systems are restricted, however, by the density of amorphous silica structures. Hence, the objective of this work was to tailor the network pore size of conventional tetraethoxysilane (TEOS)-derived silica membranes via fluorine (NH4F) doping. Fluorine doping can precisely enlarge the network pore size in a subnano range by reducing the Si–OH groups in the network structure, while CO2 adsorption properties are decreased as doped fluorine concentration increases. A fluorine-doped silica membrane (F/Si=0.1/9.9) showed a CO2/N2 permeance ratio that reached as high as 50 with CO2 permeance of 1.6 × 10−7 mol m−2 s−1 Pa−1 at 50 ⁰C. In a wet system (200–300 oC, H2O partial pressure of 3 kPa), fluorine-doped silica membranes were quite stable, irrespective of the doped fluorine concentration, but a blocking effect at around 100–150 ⁰C was observed and gas permeance (He, CO2, N2) was apparently lower than that in a dry system. Under steam with a high partial pressure (300 ⁰C, H2O partial pressure of 30 kPa), the hydrothermal stability of silica membranes was increased with increases in the fluorine concentration.