High-temperature stability of PECVD-derived organosilica membranes deposited on TiO2 and SiO2–ZrO2 intermediate layers using HMDSO/Ar plasma

• Organosilica membranes were prepared at room temperature by PECVD technique.
• The membranes were prepared on two types of intermediate layers with different pore sizes.
• The membranes with SiO2–ZrO2 intermediate layer showed good stability up to 400 °C.
• The pore structure of PECVD layer was changed by heat treatment at 500 °C.

Organosilica membranes for gas separation were prepared on porous substrates by means of plasma-enhanced chemical vapor deposition (PECVD) using hexamethyldisiloxane as a precursor. Before the deposition of organosilica membranes, two types of intermediate layers with different pore sizes, a TiO2 layer (dp ≈ 5 nm) and a SiO2–ZrO2 layer (1 nm), were prepared on α-alumina porous supports via a sol–gel method. The stability at high temperatures was evaluated by measuring the gas permeation characteristics before and after heat treatment. PECVD membranes deposited on a SiO2–ZrO2 intermediate layer were found to be more thermally stable than those deposited on a TiO2 intermediate layer. After the treatment at 500 °C, the membranes deposited on a SiO2–ZrO2 intermediate layer had a selectivity of ∼1040 for He against SF6 with a He permeance of 1.41 × 10−6 mol/(m2 s Pa) at 50 °C, although the organosilica membranes were prepared at room temperature. On the other hand, the membrane deposited on a TiO2 intermediate layer became non-selective after heat treatment at 400 °C. The temperature dependence of single gas permeance for the membrane deposited on a SiO2–ZrO2 intermediate layer was measured after the high-temperature stability test. The permeation of He, H2 and N2 for the 200 °C-treated membrane showed an activated diffusion, while those for the 500 °C-treated membrane followed a Knudsen permeation, suggesting that heat treatment at 500 °C formed pores that allowed the permeation of molecules that were the size of N2 (0.364 nm) and smaller. In addition, since the permeation of SF6 for the 500 °C-treated membrane showed an activated diffusion, the membrane should have a uniformed structure with a small number of pinholes.