Preparation of organosilica membranes on hydrophobic intermediate layers and evaluation of gas permeation in the presence of water vapor

• Hydrophobic Me-SiO2 intermediate layers were used instead of conventional layers.
• Nanopermporometry was used to examine hydrophilicity and pores size.
• High CO2 permeance was achieved for organosilica membranes under dry conditions.
• Low CO2 permeance decrease was obtained by hydrophobic membranes under humidity.

Ceramic membranes were newly developed with hydrophobic intermediate layers to achieve high gas permeance in the presence of water vapor. Me-SiO2 sols, which were prepared by co-precursors of tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) were coated as intermediate layers. Two types of organosilica sols prepared with bis(triethoxysilyl)ethane (BTESE) and bis(triethoxysilyl)octane (BTESO) were used for the top layers. By characterization of nanopermporometry using hexane and water as condensable vapors, Me-SiO2 layers showed pore diameter of approximately 1.5 nm and exhibited hydrophobic properties and that SiO2–ZrO2 layers were hydrophilic. BTESE- and BTESO-derived sols were coated on hydrophobic Me-SiO2 layers, referred to as BTESE/Me-SiO2 and BTESO/Me-SiO2, and on hydrophilic SiO2–ZrO2 layers, which are referred to as BTESE/SiO2–ZrO2 and BTESO/SiO2–ZrO2. Gas permeation was examined for four types of membranes under a dry and a wet atmosphere. Under dry conditions, BTESO/Me-SiO2 showed a gas permeation trend that was similar to that of BTESO/SiO2–ZrO2. The selectivity of H2/SF6 for BTESE/Me–SiO2 (334) was much lower than that of BTESE/SiO2-ZrO2 (>20,000) due to the inhomogeneous coatings of BTESE on the Me-SiO2 layers. Under humidified conditions, BTESE/Me–SiO2 and BTESO/Me-SiO2 with hydrophobic intermediate layers, exhibited less of a decrease in CO2 permeance compared with either BTESE/SiO2-ZrO2 or BTESO/SiO2–ZrO2, both of which had hydrophilic intermediate layers. The water vapor resulted in a negligible effect on gas permeance for totally hydrophobic BTESO/Me-SiO2, while a little larger decrease was observed for hydrophilic top layers of BTESE/Me–SiO2, showing that membranes with hydrophobic surface chemistry can effectively resist water vapor condensation or adsorption during gas permeation.