Plasma-assisted multi-layered coating towards improved gas permeation properties for organosilica membranes

Abstract

Hydrophobic silica membranes for gas separation are proposed to achieve high gas permeation properties in the presence of water vapor. Hydrophobic Me–SiO₂ sols were prepared by using tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) as co-precursors, coated on macroporous α-Al₂O₃ supports through multi-layered coatings to form an intermediate layer with pore size of approximately 2 nm. H₂O vapor plasma was used for the modification of a hydrophobic Me–SiO₂ intermediate layer by generating hydrophilic groups on the surface without changing either the bulk hydrophobicity or the pore size. After plasma treatment of the Me–SiO₂ layers, bis(triethoxysilyl)ethane (BTESE)- or bis(triethoxysilyl)octane (BTESO)-derived sols were coated as separation layers. The gas selectivity for BTESE membrane was improved after water plasma treatment, which allowed better adhesion between each layer via the enhanced hydrophilic modification by water plasma. Under wet conditions, the CO₂ permeance for both membranes were decreased, a slightly larger decrease than for membranes without plasma treatment, but much less than the BTESE and BTESO membranes prepared on hydrophilic SiO₂–ZrO₂ intermediate layers. High gas permeation properties were obtained in the presence of water for organosilica membranes prepared from hydrophobic top layers to hydrophobic intermediate layers via plasma-assisted multi-layered coatings.