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 tetrae-thoxysilane (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 x 10-7 mol m-2 s- 1 Pa-1 at 50 & DEG;C. 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 & DEG;C 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 & DEG;C, H2O partial pressure of 30 kPa), the hydrothermal stability of silica membranes was increased with increases in the fluorine concentration.

Automated summary

Research focused on tailoring the network pore size of silica membranes through fluorine doping, aiming to enhance CO2/N2 permeance ratio. The fluorine-doped silica membranes showed improved performance, but exhibited a blocking effect in wet conditions with lower gas permeance compared to dry systems. Additionally, under high steam pressure, the hydrothermal stability of the silica membranes increased with higher fluorine concentrations.