Fluorine Doping of Microporous Organosilica Membranes for Pore Size Control and Enhanced Hydrophobic Properties

Fluorine-doped organosilica membranes for gas and pervaporation (PV) separation were fabricated using a sol-gel method. NH₄F and bis(triethoxysilyl)methane (BTESM) were selected as the dopant and Si precursor, respectively, for the fabrication of fluorine-doped organosilica membranes. Doping with fluorine was evaluated for its effect on the physicochemical properties of organosilica (hydrophobicity/hydrophilicity and network size). Fluorine doping dramatically eliminated the formation of Si-OH groups in the sol, so that the condensation of Si-OH groups during the calcination process was suppressed. It is possible that fluorine doping enlarged the network pore sizes in organosilica, because the F-BTESM (F/Si = 1/9) membrane showed superior He and H₂ permeance with a low H₂/N₂ permeance ratio that corresponded to the network pore size by comparison with an undoped BTESM membrane. The F-BTESM (F/Si = 1/9) membranes clearly showed a high level of C₃H₆ permeance (>3.0 × 10^-7 mol m^-2 s^-1 Pa^-1) with a high C₃H₆/SF₆ permeance ratio (∼250), which suggests that the network pore size of F-BTESM is suitable for the separation of large molecules such as hydrocarbon gases (C3/C4, C4 isomer, etc.). Organosilica membranes both with and without fluorine doping showed stable PV performance because of the fact that H₂O permeance and each permeance ratio under different separation systems was approximately constant over 10 h at 70 °C. Fluorine doping enhanced the hydrophobic nature of the organosilica, which was confirmed by the H₂O adsorption and PV properties.